Do you have a blackberry
data plan on your mobile service account? (If you don't, you soon will as I understand AT&T automatically adds it when they dectect a smart phone on the network.)
Anyway, if you have a blackberry data plan, resend the service books. See How do I resend my service books? - BlackBerryFAQ
If you don't have a data plan, you may or may not be able to use the wifi browser, I don't know. But Open options on the device. Then Advanced Options. Then Browser. Set the Default Browser Configuration to Hotspot Browser (or it may be WiFi on your device). Then try again.