I had trouble finding this so I'm assuming others may as well. It's a tiny pin-sized magnifying glass icon in the top middle of the forum page.

I win and my forum threads are outstanding and interesting. Join the Cool Person Club.

Hmmm... I never thought to look for it.

Also...

I had the same "oh, there it is" experience. I was already thinking about writing to the website support to complain about the exclusion of such an important feature, and then yesterday my eyes strayed to the top of the page and there it was! Quite awkwardly designed.