if you are using my flash and MMS is not working, it indicates it is most likely a Sprint based rom. So far i have not found a solution as to why Sprint based roms do not send MMS properly.
If your on a hero my only suggestion is to use the rom DamageControl 1.0 or DamageControl 1.1 which are eris ports and should work flawless. The issue comes down to the rom not supporting MMS properly on any network other than sprint's.
if your on damagecontrol 1.0 or 1.1 and you used my flash and your not getting internet it sounds like you dont have the autostart.apk installed. MMS should still work though, if you used a previous flash of mine, try the newest one, v10 for cricket, it should get you all setup, my previous flashes had MMS going through the proxy, which caused issues. MMS should work fine even without internet if your apn is set correctly, which from above should work fine, except on sprint roms, which is why i reccomend damagecontrol 1.0 or 1.1, which is based on a port from an eris, which works fine. Sprint does something weird to the MMS that we are still trying to nail down.