the ds requires
802.11B (on most routers b/g split mode will work just fine)
no encryption or wep 64/128 encryption (only)
channels 1, 6 or 11 (as recommended by nintendo)
ssid set to public broadcast
if your router is using something different you won't get your nds online. if you are using mac address filtering make sure you have the mac address of the nds added. if all this is set then double check the settings inside your nds. you will need to add the wep key to your nds. also you may or may not need to configure other aspects manually. some routers have issues with having plug n play, smart packet interface firewalls, and range enhancing features when it comes to the nds. so double check all of those as well.
if all checks out try some retail games with wifi, some roms, some homebrew and then post back with more problems.
-another world