If you're on a BES the app will work with no modifications.

If you're not on a BES then you have to add the proper apn into the TCP settings and check the box in BBWeather that says "disable MDS proxy."

Go to Options >Advanced Options >TCP. For TMobile the apn should be:

apn:wap.voicestream.com
username
password

leave username and password blank.

This is all in this particular thread.