Also, I saw this on another forum, tried it and it didn't work:
"I had the same problem on 2 different versions of CM on 2 separate phones. This fix worked both times. Use root explorer, and make sure it is set to R/W mode (read/write). Navigate to /data/data/com.android.providers.telephony/databases/telephony.db and delete that file, then restart your phone. After the restart, you should be able to send and receive MMS messages."