When in the forum, you will either see the "find message" or the "back to forum" bottom at the bottom left of your screen.    If you are reviewing a particular thread, then you won't see the "find message" button, but the "back to forum" button. Click on "back to forum" and then you can use the "find message" button.  If your search doesn't find anything, make sure you are setting the # of days back to search greater than 30.  30 days is the default of the search.

-Hoot.