Well I'll just warn you ahead of time that the "wifi engine" some carts advertise has nothing to do with playing games online.
It's generally used for...
A - Downloading ROMs right to the cart. We don't support warez sources so we can't really help you with that directly. Also DS Wifi speeds suck total ass, it'll be a lot quicker to just download on a PC and transfer to the MicroSD.
B - Downloading software updates. These tend to stop working after a bit because they need to replace files that can't be replaced while being run, so sometimes you need to resort to the PC method anyways.
C - Downloading extra data like cheat databases. these tend to actually be the stable thing, but they tend to download outdated databases.