1. per the HH website, wait 3 weeks for the stay to post before doing anything else.
2. On the HH website there is a link for reporting missing stays. They ask for things like the amount of the bill and how it was paid. Easiest if you've kept your receipt. Then you wait awhile longer (3-4 weeks more in my experience).
3. I beleive you can call the HH service center (though I never have) and talk to someone in person. They may ask you to fax your receipt. Apparently this closes the issue faster than via the website.