If you can't manage to get going with Amir's advice then you might have a driver problem.
I had problems when 3.0 beta came out and used one of the Windows drivers off my Windows disk that came with the wireless USB. You might have to try a couple till you get the right one to work (that is either XP, Vista or Win 7 depending upon which drivers are available for your card). You could also try a wireless USB like I did. They don't cost much and the latest one I bought for an old Dell desktop works just fine.
Anyways, the problem may be the same with your set-up. Go to the System\Admin menu and down to Windows Wireless Drivers and see how you go from there. Oh, you will probably have to extract the drivers using Windows to get the .inf files.
Good luck.