You can find instructions on how to jailbreak your iPhone here: Windows, Mac
IMPORTANT: This patch will ENABLE the MMS feature on the 3.0 firmware. It will NOT input the correct settings for your carrier. Therefore MMS will only work once the correct carrier settings are set.
Press to launch Cydia from your SpringBoard
Press to select the Sections tab at the bottom of the screen
Press to select Messaging from the list of Categories
Press to select ActivateMMS2G from the list of Packages
Press the Install button at the top right of the screen
Press the Confirm button at the top right to begin installation.
Once installation has completed successfully press the large Reboot Device button.
You iPhone will now be automatically restarted. Once the reboot has completed tap to open the messages application from the Springboard.
Notice you now have MMS capabilities. Please be aware that your cellular provider may charge an additional fee for using MMS. Please contact them before send an MMS to verify charges.
Thanks go out to geniusan the author of this patch! You can learn more about how to use MMS here
To input your networks settings please navigate to Settings - General - Network - Cellular Data Network Settings