Magic,
If you're using the RC from this year, you can simply go to IFI's website and download the default code (
http://www.ifirobotics.com/rc.shtml, scroll all the way down to the bottom, it's the link called "2007 RC Default Code" or something) and this provides the functionality you were asking for by default. You have to plug the PWM cables into PWM 1 and 2, and have joysticks in port 1 and 2 on the operator interface.
To download the code onto the RC, you use the same link as above to get IFI loader. You then stick a serial cable into the program port on the RC and a serial port on your cable, turn on the robot, and hold down the program button until a little orange light (I believe by "program state") turns on, then you just have IFI loader download the "FRC_Default.hex" file.
What other functionality are you looking to add? If you're looking to reduce the sensitivity or compensate for one side being mechanically stronger than the other, we'd need more details to help.
If you have any further questions or specific requests, you can always PM me.