The M3DS Real is a flash cart for the Nintendo DS manufactured by the M3 Team. Flash carts allow users to store and play backup copies of their games and home brew (user-made) rom files. As a slot-1 flash cart, the M3DS Real fits into the same slot as normal Nintendo DS games, and it cannot run Gameboy Advanced (GBA) roms. To properly run their GBA backups, users need to purchase the GBA Expansion Pack designed for the M3DS Real, which fits into the Nintendo DS' GBA slot.
Connect your Micro SD or Micro SDHC card to your computer through either your computer's integrated card reader or the USB card reader supplied with the M3DS Real. Open the NDS folder, which contains your Nintendo DS roms; the GBA roms go in the same folder.
Locate the GBA backups and/or home brew roms you'd like to add to your M3DS Real using a separate window. Drag and drop the files onto your Micro-SD card by clicking them, holding the mouse button down and dragging them to the M3DS Real folder and letting go.
Put the M3DS Real cartridge into slot one of your Nintendo DS, putting the GBA Expansion Pack in the GBA slot on the bottom of the DS. Turn the Nintendo DS on and wait for the main M3DS Real menu to load.
Scroll down to the GBA icon and either tap it with your stylus or press "A" to open it. Locate the GBA game you'd like to play and press "A" to load it.
Downloading roms of games you don't own is illegal piracy and should be avoided to prevent any possible legal action by the content owner.