Did you do the system requirements test for the USB dongle (on the website somewhere, I think)? And when you plug it in, is it recognized, the program runs in the item tray, the icon is blue, the USB dongle light lights up etc.?
At any rate, there's also at least one Nintendo wifi forum that you could try asking at...they have a ton of info. I've had my USB adapater stop working a few times, but reinstalls always fixed it.