This has been educational, like it or not!
I didn't know about the drop-down either, just the squiggly lines. (Firefox)
I don't really know what to look for either, I never posted enough (before the upgrade) to know about a spell check button or anything like that.
I looked into an add-on, and found one that uses a different (commercial) server, and they evidently put ads up in payment for checking your words. A similar one is in Russia, that will spell check your English, Russian, Slovakian.... That's about it. I guess spell check is a complicated enough thing that it's not found in the (extensive) list of forum mods available.
The forum *does* have spell check built in, but it needs a particular library on the server, and I have a support ticket in to find out about that. I don't think they do.
======= Edit: They don't
Dr. Goggles, I just added "*-Tunes" to the list of banned words.