It will 'not' work if you use a PAL Animal Crossing : City Folk, will only work with a NTSC (American) DS game, If you have a PAL game, so you will have to wait for the PAL City Folk.
It also does not work with Flashcarts that use roms. You will have to have the official cart.
If you have both NTSC and the DS cart is original then seriously follow the onscreen instructions, it tells you 'word for word' how to do it,it does not just say 'is the ds connected' it says exactly how to do it.