1) Make sure you have the data package on your line.
2) You can try to edit the MMS Server settings by going into the Options of MMS.
Change to the following or ADD a new server and set to the default:
Verizon's server address:
http://mms.vtext.com/servlets/mms?X-VZW-MDN=0123456789
(replace 0123456789 with YOUR phone#)
Port: 80
WAP version: none
3) Get a newer phone!