How do I enable multimedia messaging on my unlocked Samsung Propel?

I have an unlocked Samsung Propel from AT%26amp;T but I have t-mobile everything works fine except for the multimedia messaging for example, when I try to send a picture it says "sending failed service unavailable" how do I fix this?How do I enable multimedia messaging on my unlocked Samsung Propel?Ok i had the same exact problem, but all you need to do is call the samsung support number and they talk you right thru how to fix it, sorry tho i dont have the number on me. let me kno if this works %26gt;%26lt;