To enable MMS, you have to connect the iPhone to iTunes and click "Check for Update." You'll receive a notification that "an update to the carrier settings" is available. Click on Download and Update, then restart you iPhone after the update completes.
Update: I had a problem with updating my 32GB iPhone 3GS to the new MMS carrier settings. Immediately after restarting, my iPhone displayed the dreaded "connect to iTunes" screen and iTunes gave an error that it "cannot read the content" of my iPhone and to restore it to its factory settings in iTunes.
Did the MMS update work for you?