You have to sign up for a wireless account by going to the IT services desk which is on Lvl 2 in the main library (the level that you walk in from). The link is
here.
Mind you, the wireless services isn't that great in the Law Building for some reason. Your best bet would be the library and some of the lower classrooms - the upper levels tend to fluctuate. Also, I would only use the wireless services for uni based websites because of the charge - if i'm accessing external websites I usually go down to the law library and use the computers there.