The common problem with the WIFI usb is that it never works. It connects all right, and then in the connection test, after two or three seconds, its gone. This happens almost only in wireless internet connections. I'll tell you how to solve it.
Whenever a computer connects after booting, in a wireless connection, it receives an IP adress. In wireless connections, this constantly changes. Once or twice you may get a lucky hit. When the USB installs, it gives the IP adress to nintendo for verification. If your PC tries getting in after rebooting with the USB, it fails because nintendo does not verify the IP.
My solution was to reinstall the USB everytime you want wifi. This may not sound very practical, but solves the problem and after this worked for me all the time. If you use wifi a lot, this might be tiresome, but deinstalling and reinstalling everytime is ok since it takes only about 15 minutes. So if you use it only a little, this might help you a lot.