If you can't remember the title or author, then posting to the search threads is often the best beginning. Someone might remember a partial title, or think they know the author.
If you want to search yourself, at the top of each subdomain is a menu. The Member Tools menu includes a link for "Search" where you can look by author, title or summary. If you are including words under four letters long, enclose them in quotes to make them searchable. We have a FAQ for the Search function here:
http://www2.adult-fanfiction.org/forum/index.php/topic/33784-how-to-use-search/
As far as marking a story as a favorite, we do not have that specific feature. What we do have is the Currently Reading and Recommended Reading portions of your archive profile, which you can edit in the control panel. You can add stories to those sections, and it creates a link to the story, making it faster to find.
The FAQs are here: http://www2.adult-fanfiction.org/forum/index.php/forum/790-profile-how-tos/
If this is an Original work, please keep in mind that many of our authors take stories down when they are about to publish them. If that's the case, it might have been noted in the author profile. Obviously, without more details, I can't check on my end.