This video will show you how to enable MMS on your iPhone 3G and 3GS on 3.1 firmware. First, copy and paste the terminal code. Hit enter. Go back to the description and click the link to the file. Connect your iPhone and go into iTunes. Hit options, check for updates. Go to where you have downloaded the program.
You will see it being uploaded. Take your iPhone and reboot it and you should then have the MMS icon enabled.
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