Go to Options>Advanced Options>TCP, then enter wap.voicestream.com for the APN setting.
Or go to Options>Advanced Options>Host Routing Table>Register Now. This will send the Browser, Download Fun and Setup Internet Email icons to the phone.
You must have a Blackberry Data plan in order to use the data reliant features of the Blackberry. Other GPRS/EDGE data plans from any provider mean nothing.
If you want to use the data portion of the Blackberry you'll need to call T-Mobile and add a Blackberry plan to your account.