LG Xenon unlocked help, please? MMS will not send.

I bought the LG Xenon unlocked from craigslist and the man assured me that he had no problems with this pho ne at all. It ha worked amazingly, great battery life, nice keyboard, and I haven't had any problems with the touch screen. However, I cannot receive or send multimedia messages and I cannot sign on IM. Whenever I try to send mms, my phone shuts off. I am on T-Mobile. Is there any way to fix this?   

Please and thank you.

It is supposed to be for AT&T. I found on instructions T-Mobile's website for changing it. I did change it, and when I tried to send a message, my phone turned off again.
I'm not sure if it's a problem with the center/gateway or if it is just the phone. Should I call AT&T or T-Mobile for help? And would they help me, even though my phone is unlocked?

Thank you for answering.