I have had the XV6700 for a couple of months now and have had the same problem.. When I would attempt to send an MMS message I would receive an error saying that the Message Failed--Retry Now/Retry Later..

I just called text support and got it working by doing an over-the-air software update...by dialing *22899 <send>

Also, as some of the other posters have noted--when selecting a recipient for an MMS msg--you have to tap the "To" to access your Contacts..not the empty field --but actually tap on the word "To".. For some reason it's kind of difficult.. I have to use the stylus to hit the small target, instead of my finger which I use for everything else.

Hope u get it worked out!