As for how to Set up MT-Card 3DS to Play multi-rom/3DS Games, you can follow the steps below:

Preparation:

(1) MT-Card (blue card + yellow card)

(2) 3DS/3DS XL/LL (system version 4.1.0–4.5.0)

(3) Micro SD card (4GB/8GB/16GB/32GB)

Step1: Set up the Blue Card
A) Format the MicroSD card (FAT32)

B) Download “Blue card file” [Download], unzip and open the folder, copy all files to the root of the MicroSD card.

Step2: Run MT_INSTALLER.NDS (repeat on every DS mode entry) .

A） Insert the MicroSD card into the blue card and put it into your 3DS/3DS XL. Then press KEY A go to the menu of BlueCard;

B） Running MT_INSTALLER.NDS in BlueCard menu, and selecting your console type, then press KEY A to exit after finished.

Step3: Prepare the SDcard of 3DS/3DS XL console

Download the “SD card file” [Download]. Extract and open it, copy Launcher.dat to the ROOT of the SDcard of your 3DS/3DS XL console (version 4.1‐4.5 only).

Step4: Run Launcher.dat (repeat every power cycle)
Choose “System Settings”‐>”Other Settings”‐>”Profile”‐>”Nintendo DS Profile” on the Menu of
3DS console, and the console will restart soon.

Step5: Copy games (and saves)
A) Prepare the Yellow Card and a MicroSD card (Sharing with the BlueCard is OK), and format the MicroSD card as FAT32;

B) Copy 3DS ROMs (*.3ds) and SAVEs (*.sav) to the ROOT of the MicroSD card. (The MT‐Card will create a 512KB .sav for you if it can’t find a save file with the same name. BTW, the 4GB ROMs please trimmed first. )

Step6: Menu operation
A） You can enter the menu of Launcher.dat by pressing KEY L when choosing “Nintendo DS Profile”, and you can use KEY LEFT
and RIGHT to select functions (Pressing KEY A to choose, the other functions are being developed, coming soon);

B） You can choose “Go Menu” return to themenu of console, and you can quick return by pressing KEY R anywhere also;
C） You can choose “Select Game” to select the ROM to run, pressing KEY UP and DOWN forselecting file,
pressing KEY LEFT and RIGHT for page up and page down, and KEY A for OK, KEY B for cancel.

Step7: Switching games by pressing KEY
The MT‐Card has a KEY build‐in, you can switch the game by pressing the KEY on the menu of 3DS console.
It will be locked when starting the game, and will be released after exiting the game by pressing KEY HOME.
Please pressing the KEY again after the game icon display is completed. It will lead to the console disorder if
you pressing the KEY very quickly, please re‐insert the MT‐Card in that case.
Step8: About the SAVE
The MT‐Card saves the game SAVE to the .sav file with the same name at the same time. It is necessary to
pressing KEY HOME to backup the game SAVE, you can even pull out the MT‐Card after saved in the game,
and you can backup the game SAVE by copying it to PC also.

Attention:

What are the new features the Gateway team brings us? On the one hand, the GW firmware 2.0b2 can support the latest 3DS emu-nand V7.1.0 (Pay attention that it is the emulated 3DS system, not the real one.). Gateway 3DS card can only be compatible with real 3DS or 3DS XL system ver.4.1—4.5. But with the firmware 2.0b2, you can play 3DS games with ver. 7.1.0-14.

Gateway 3DS firmware stability is also improved. Then what about another 3DS game card, the R4i gold 3DS Deluxe Edition? According to its official news, it is said that its multi-rom function will be released with next firmware updates. More information, just focus on www.dwtechz.com!

As an official reseller, Dwtechz.com provides genuine and original R4i 3DS cards for Nintendo games funs.
The best r4i 3DS card, R4i gold 3DS, is only US$ 17.20.
R4i-sdhc 3DS rts is only US$16.00.
The cheapest one is Ace3ds Plus, only 9.49USD.

The price of some hot products are also cut down. Cobra ODE is only US$ 123.00.
R4i gold 3DS Deluxe Edition is the cheapest 3DS card to play 3DS games. Only US$ 56.90.

Gateway 3DS is only US$ 77.80 now.

Do you believe it? Then go to www.dwtechz.com to have a look!

1. Download the Wood R4 kernel V1.64
2. Extract it and open the folder
3. Drag the files “_rpg” and “_DS_MENU.DAT” into the root of your MicroSD card

(Note: If you have had these two files on the memory card, just replace them with the new files.)

Then you can play DS games with the new kernel! If you prefer to play 3DS games, you can turn to R4i gold 3DS Deluxe Edition, the latest 3DS flashcard from r4ids.cn for 3DS roms.

The R4i gold 3DS Starter cards, the deluxe edition, include two cards inside. The R4i gold 3DS starter card is compatible with 3DS v4.1.0 to 4.5.0 in all regions. Another card is actually the R4i gold 3DS card. It has all functions or features of R4i gold 3DS. It can work on 3DS v6.3.0-12 on US, Europe etc. versions. It can also compatible with DSi, DS and DS Lite.

R4i gold 3DS starter cards are sold cheap on Dwtechz.com. It is less than US$70.00.

Things we need:

1. 3DS/3DS LL/3DS XL (System version should be 4.1—4.5; No region versions limit; If your 3DS system version is lower than 4.1, you can take the Luigi’s Mansion to make your console upgrade to 4.5)
2. TF/SD card (4GB is ok)
3. Card Readers for TF card and Micro SD card
4. Downloaded 3DS roms

How to Setup / Install Gateway 3DS card

Phase 1:

1. Format your TF/SD card (with Fat32).
2. Go to Gateway 3DS official site http://www.gateway-3ds.com/downloads.html to download its 1.0 firmware.
3. Extract the file, open the folder, drag all files on the “Blue Card (R4i)” folder into the root of your TF card
4. Insert the TF card into your gateway blue card.
5. Turn on your 3DS console and put the blue card into your console. Click on the R4i icon (“deep labyrinth”).
6. Choose the “GATEWAY” file and install the gateway mode. After installing successfully, there will be a tip to press “A”. And then turn off the console. Take out the blue Gateway 3DS flashcard from the console.

Phase 2:

1. Take out the SD card from the 3DS console. (Note: The SD card is from the 3DS console, but not from flashcart.).

2. Open the unzipped folder “GW_Release_1.0” (which is downloaded from the beginning). Open the “GW Release” and choose “Launcher.dat” and copy it into the root of that SD card.

3. Insert the SD card back to your 3DS console and turn on your 3DS

4. Enter the “System Settings”→ “other Settings”→ “Profile”→ “Nintendo DS Profile”

Wait several seconds. Then the 3DS console can boot Gateway mode.

Phase 3:

1. Take out the TF or SD card from the blue Gateway 3DS card. Format the card.
2. Download a software Win32 Disk  Imager: http://sourceforge.net/projects/win32diskimager/files/latest/download
3. Boot the software and then choose the “Device” on the up right corner. Then click on the left files icon to choose your 3DS roms. (“.3ds” roms).
4. Insert your TF card to your computer, click on the “Write” to copy your games into your TF or SD card. (Note: the capacity of your memory card should be larger than the rom)

After that, take out your TF/SD card. Insert the TF/SD card into the red Gateway 3DS cart. Then insert the red Gateway 3DS to your 3DS console. Then you can enjoy your 3DS rom!

It said that the blue card, with DS mode, can work on 3DS V6.1.0. It can be used for DS backups and running DS home brew. The custom Gateway Installer DS app can be run once to setup. Thereafter your console can used in DS mode with the blue cart.

Can all 3DS Games be played with Gateway 3DS Card?

As a matter of fact, not all games can be played with the red cart of Gateway 3DS card. Animal Crossing and Mario, released last week, can not be compatible for now. But the Gateway 3DS team is trying to improve the card to be compatible with more games. The red 3DS card can work on 3DS firmware version 4.1.0 to 4.5.0.

The Gateway 3DS card will be available soon! More details, you can focus on http://www.dwtechz.com/gateway-3ds-flashcart-for-3ds-xl3ds-playing-3ds-games-p-17454.html.

Quote from http://gbatemp.net/threads/gateway-release-new-video-first-units-to-ship-this-week.351334/.

Are you looking for cart which can play 3DS games! That’s it! The Gateway 3DS card is the only flashcart which can play 3DS roms. From the letter from Gateway-3ds, its first unit of gateway 3DS card will come in this week. You can keep an eye on http://www.dwtechz.com/gateway-3ds-flashcart-for-3ds-xl3ds-playing-3ds-games-p-17454.html .