uhh how do you play pc games on ds?
They're playing the GBA version of the game, likely using GBARunner2.
On DSi you can do this with software mods, or using a flashcart (R4i, etc.)
On DS / DS Lite, softmod isn't an option, but you can also use a GBA flashcart such as the EZFlash Omega or EverDrive GBA X5.