After some fiddling to get MMS working, this is my complete APN setting for AT&T Wireless (blue) mMode:

Name: mMode
APN: proxy
Proxy: 10.250.250.54
Port: 8080
Username: <Not set>
Password: <Not set>
Server: <Not set>
MMSC: http://mmsc.mymmode.com
MMS proxy: 10.250.250.55
MMS port: 8080
MCC: <unchanged>
MNC: <unchanged>
APN type: <Not set>