According to some folks over at dbstalk.com, the AM21 option will only show up on the DirecTV web page if you actually have an HR21 HD DVR on your account. This means that anyone that does not have the HR21 will not have the option to purchase the AM21 add-on tuner module. It apparently does not work with the H21 receiver.
Please don't PM me or any other members looking for personal assistance. You'll do better by posting (after you've exhausted the search feature, of course) and taking advantage of the collective expertise of the membership instead of a single individual that may or may not be able to help you. Thank you and enjoy your stay at DDB!