try these apn settings. once you have entered and saved them, apply them by tapping the radio button next to the apn name. and also make sure you delete all other apn's. reboot your phone and give mms a whirl. if for some reason it still isn't working, try changing mms port to 8080, reboot once more and try again.
i'm using these settings with at&t gophone on my n4, and although i do have a data plan, i don't see why they wouldn't work without one.
MMS Proxy: proxy.mobile.att.net
MMS Port: 80
APN type: default,supl,mms,hipri
APN protocol: IPv4
APN roaming protocol: IPv4