Two of my cars have homelink based remote controls.
Homelink is at this site.
I had tried several times on my own, but do to the rolling code security model used by my garage door system it never worked.
Here's what might work if you have a security plus system (so secure you can't work with it):
1. if you have three buttons built into your car, hold the two outer ones until the light blinks (20-40 seconds) - this will reset your car.
2. take the ugly garage door remote and hold it close to thebuilt-in buttons (near your car's rear view mirror).
3. hold simultaneously, the garage door remote (clicker) and press one of the three built in buttons. Keep holding them until the light blinks (in your car near the buttons). you have just programmed the homelink button.
4. step out of your car. find the training button on your garage door opening mechanism. you may need to remove the plastic cover if it has a built in light to find it. you may also need a ladder to reach it.
5. push the opener's training button - on my machine it's red.
6. you now have 30 seconds to go back to your car and push the bult-in homelink button you just programmed three times. you should hold the button for 4-6 seconds each time.
7. when done hold the homelink button in your car for 3-6 seconds. your garage door should be activated.
it's a good idea to do this with your car enginer off.
have a great weekend,