Apn settings ? Galaxy 3
Go to Settings > More / More Networks / Wireless & Networks > Mobile Networks > Access Point Names > New APN setting (or a plus sign)
Enter this:
Name: Public Mobile
APN: sp.mb.com
Proxy: Leave blank
Port: Leave blank
Username: Leave blank
Password: Leave blank
Server: Leave blank
MMSC: http://aliasredirect.net/proxy/mb/mmsc
MMSC proxy: 74.49.0.18
MMS port: 80
MCC: 302
MNC: 220
Authentication type: Leave blank
APN type: Leave blank
Hope that helps!