1) press the two outer buttons until the light flashes. This clears all previous codes
2) press and hold the button on the Homelink transmitter you want to program, it should flash (~once per second).
3) while still pressing the button from step 2, hold your original door opener near the homelink transmitter, and press the button you want the transmitter to learn.
4) when the Homelink transmitter correctly receives the signal to learn, the light will flash faster (~twice per second).
That's just off the top of my head. If you need more info, just register your car (for free!) and download the owners manual from
http://www.mygmlink.com/