@craig675 First off, you should realize that the connections used for DSi games/apps are separate from the connections used for DS cartridge games. The former allows WPA-encrypted access points, while the latter does not. But from what you've said, you're using an access point with a WEP-encrypted password anyway. If your network requires you to manually enter everything, you'll have to do that and carefully, I might add. If not, you'll want to set IP and DNS to 'auto-obtain'. Manual config isn't really recommended, so you might also want to check up with that on your router.
@SBP Tonex Any DS game (that doesn't require the GBA slot) is playable on the DSi. If the online mode to a particular game doesn't work, it is due to the card's broken/corrupted online config, not the system.
OMG DUDE I HAVE THE SAME PROBLEM!!! I tired everything... Mario Kart, Animal Crossing, Pokemon, The Legend of Zelda... NOTHING will work on my Nintendo DSi online... BUT my Nintendo Wii works PERFECTLY online... D< I need help TOO!!!