Yes, I’m using it on my Mac. However, to get it to work, I had to enter it manually. To do so, go into Preferences/Key Commands, type System Text into the Search field, which brings up NoteInput/Create System Text, and then assign the Opt-Shift-X keystroke (or whatever you want—I used a different keystroke) to Create System Text.
The fact that you get a “can’t do that” beep when you try to use it makes me think you may have Opt-Shift-X already assigned elsewhere. If so, when you input that keystroke to trigger Create System Text, you’ll get a message telling you where it’s already assigned, so you can deal with it.
Daniel wrote in another thread that right now, any new keystroke commands issued in version updates don’t go into effect for users who have already assigned even one keystroke command of their own (I have many and had to deal with this.) It sounds as if you may have at least one that you’ve assigned, so none of the new keystrokes the Dorico team included in v. 1.1 will be in effect; you’ll have to input them yourself. He said that problem will be addressed in future version updates.
[Sorry—Andgle answered this part of it while I was still typing.]
Hope this helps.