You need to add the Sprint server address. From sms/mms go to menu then pics/video options. Select the CDMA Servers tab and select new:
Server name: Sprint
gateway:
Port Number: 80
Server address:
http://mms.sprintpcs.com/servlets/mms
Connect via: The internet
after that click done, long press on the newly created sprint and select set as default. My wife sent me a mms and it worked fine.