Excellent advice! Wish I had learned that 2 years ago.Just want to save others the trouble from the aggrevation of searching online and follow all the instructions that didn't work. The following post by member "burned300" http://www.toyotanation.com/forum/showthread.php?t=199685
is the only one that actually worked on my 2008 Sienna with my Liftmaster Professional Garage Opener because of one minor, but very important detail, that even the Owners Manual got it wrong (or applies to other brands, but not Liftmaster).
Almost all the instructions from all the forums were correct in general: First, you must "sync" the car's Homelink button to your Liftmaster handheld garage opener. Second, then "sync" the Garage Opener head unit with the car's homelink button.
The only tricky part is the that the owners manual and all the other posts said to "Hold the Homlink button AND the handheld transmitter simultaneously for up to 60 seconds". In my case with the Liftmaster, as "burned300" should receive the credit, you have to press the car's Homelink button by itself for about 15 seconds until it starts SLOW FLASH, then, WITHOUT releasing the homelink buitton, press the handheld transmitter simultaneously until the Homelink button starts FAST FLASH. Once this is done, the proceed to the next step, which is very straight forward.
So, if your Homelink don't work, follow the above link to "burned300" posting.