Need serious help with connecting to the internet using a wireless adapter..anyone?

Right, so its quite along story. My dad recently signed up for o2wireless broadband, has installed the rooter that comes with it, and is successfully connected to the internet using the ethernet cable (following instructions on the o2 cd). But when he tried to connect using a wireless adapter, geez, it does nothing. I've literally tried evrything i could think of. using the cd instructions, they sed it wudnt connect to the network, then i tried reinstalling the adapter (thompson one) and that just did nothing. i rekon its something to do with the settings on the adapter not matching the rooter's, but i have no idea how you change them, i dnt no if any1 can help, but and suggestions realy wud be amazin. Im not much of a computer whizz, but i no enuf to no that this is just ridiculous tbh lol. bin 2 weeks now. He even got a new adapter, and a new rooter, which has had no effect. I hope some1 can help with this conundrum!