A couple of things come to mind. First, my memory is that the threat window hotkey is CTRL-T by default. I've never had a problem getting the threat window to pop with that. I think the setting will toggle whether you're in combat or not, but it's a lot easier to tell that it works when you toggle the setting while in combat. I have seen the window show up in odd places. It *might* be hidden under something else. It can be hard to track that down.
If it were me, I'd pick a fight with something that can't hurt me, and then ignore it while I played with windows. Hitting F10 once will highlight all windows so you can see them (also unlocks them temporarily as of a recent patch).
-= Therendil =-
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