Have you scanned for the network and created a profile for it, or if it's only a temp hotspot set it as such for a 24 hour connect? If so the Wifi icon should go white, instead of grey, which means you have a full BIS (or BES if you're that way inclined) connectivity. The browser should then default to that and switch automatically to internet when you lose connection. If the icon stays grey you have a provisioning problem on your account and that depends on the network, but I'd assume all operate the same.
Ring them and speak to some f**king 'tard, tell them to put you through to the BB provisioning team. Then when you get someone that actually has half a clue tell them that your account has been screwed up and you need the provisioning changing as you can't use Wifi. You could also try resending the service books first though as they will usually ask if you've done that.