I have a LG Quantum WP7 with Metropcs and I like the phone. After searching the web, installed the LG' "Network Setup" and still didn't have luck.
Finally I had the following set up for APN. Now I can send / receive MMS.
1. go to "Settings"
2. select "cellular"
3. add or edit apn
4. enter the following for Metropcs.
APN: fast.metropcs.net
and leave the rest fields in blank.