Whose wifi? You just can't get on anyones wifi unless you log onto it. Make sure in settings that wifi section is turned on and also ask to notify you when in range of a wifi if you want to join. But they are always password protected.
Well then what are the security settings on the Router? You have to go into settings, under wifi and click to join or it should discover on its own and ask you to join, then enter your security settings.
Haha just checking, sometimes it seems as if the most obvious things are the ones you overlook the most. As the others have said, I would make sure that there are no firewalls associated with the router on the computer it is connected to and then making sure you enter the Network Key in exactly, try and be case sensitive specific as well.