check ou the archives, there are some very detailed instructions.
in a nutshell.....
reset the homelink thing first.
then hold down a button till it flashes.
Press your original garage door opener.
once the homelink has learned your code, you have to get your main garage door opener unit to learn the homelink code.
so on the main unit, press your "learn button" whatever that is. and then run down to your car and press the homelink button.