Now, MMS should be enabled.
Go back into the messaging app: When you compose a message, you should now see a camera icon underneath the body of the message. Tap that to add a picture or video to your message.
Also, when browsing photos and videos in your photo library, you should now see an option to send the photo or video by MMS. Previously, the only option for sending photos was via e-mail.
Congratulations! Your iPhone is now capable of sending and receiving picture and video messages. Enjoy.