Well, you're only going to get 2G service using an unlocked T-Mobile device on AT&T. Do you have an active data plan with AT&T? Have you had the AT&T service books sent to the device and had it registered on the AT&T network?
Try an HRT and then a Resend of the Service books, and finally a battery pull. For the HRT, Go to Options (the wrench), then Advanced, then Host Routing, then press the BB button (menu), and select Register Now. Then for the resend, go to the Setup Folder (or it might be on your homescreen), and go to Email Setup, then Sign On (you will need your user name and password, if you don't know it, then you need to contact your carrier), then press BB (menu) and select Service Books, then Send Service Books. Finally pull the battery. This should fix you up, if not then call your carrier and make sure you are provisioned for BB data. You MUST have a BB data plan.
