Scratch that!
I called back and spoke to someone else. This lady knew what she was talking about.
She had me go into Cydia and search "Legacy." Two things should come up. Both start with "TMobile." Download and install the first one.
Soft reset the phone.
Go into the Cellular Data Settings and make sure this is under MMS:
APN: wap.voicestream.com
Username: (blank)
Password: (blank)
MMSC: mms.msg.eng.t-mobile.com/mms/wapenc
MMS Proxy: 216.155.165.50:8080
MMS Max Message Size: 1048576
MMS UA Prof URL:
http://www.apple.com/mms/uaprof.rdf
This information should have been entered automatically by Legacy
Have someone else send you an MMS. They told me that if it didn't work to retry a few times and it eventually will.
I just tried it and it worked for me!