Unlocked Samsung Comeback not sending MMS messages?

[shorty]

i have an unlocked Samsung Comeback. its originally from TMobile, but im using it on my AT&T plan. It wont send MMS(picture,ringtone) messages. How can i get it to do this? every time i enable the data roaming and try to send a message, it says, "data charge will occur. enable data transfer while roaming?"
so how can i get it to send MMS messages?
please give details. i just got the phone and i'm not used to the way tmobile phones are set up yet

[Selfchick]

It has to be configured to at&t try looking on their website for configurations or you can call i know tmobile provides them for unlocked at&t phones so i'd think it'd be the other way around also.