Try this - go out to the main forum page then enter the General Discussion forum. Right under the Facebook Link is the page listing of threads.. right below that it says Forum Tools and Search Forum - try the dropdown that is Search Forum and see if the Advanced Search in there. I just tested and that's one of the places I'm seeing it. If you don't see it, maybe try clearing your cache and reloading? I had some oddness going on myself for a bit where some of the forum options were messing up until I cleared. (it will default to viewing all threads etc for that forum, but you can choose a different one)
If that doesn't do it, i'm at a loss besides to say the issue may be related to your computer or browser.
 hey neat, I found another way to get to it. Within a thread, if you go to the dropdown that is "Search Thread" there's an advanced search option there, it also has the ability to search other forums for the threads started by X, etc. Try that maybe, should see that at the top of this page..