Seems to have disappeared just like penny candy! Try customer support on the Marriott.com web site or better yet, call Marriott Rewards and ask them.
So, here is the no longer valid listing from the Marriott Rewards FAQ page
Send us a copy of your hotel folio (bill) and your Marriott Rewards account number to the Marriott Rewards Customer Support location nearest you. You can print a fax-in “Missing Stays” form for by signing in to My Account and clicking on the link for missing stays.
However that link no longer exists, sorry!
The missing stay form shouldn't be this difficult to locate. It needs to be moved or at the very least added to the "Account Activity" page.
Hope this helps.