Did you ever get your downloaded sounds working? It's now Dec 12, 2020... so it has been quite a few months since you filed your question back in May.
The reason I ask is that I am having the same issue. For me, I have downloaded the guitar bundle and installed it. The install procedure says installation was successful. ... Yet when I assign Acoustic Guitar as an instrument to a track and selected "Acoustic Guitar" (Ac. Guitar) from the Norton Instruments dropdown menu, not a peep comes out during playback.
Also, when I close the file and reopen it, I always get the popup warning "Ac. Guitar not available".
This is soooo frustraiting. I've been trying for at least two weeks. Like you I tried everything I could think of including completely rebooting my computer... but nothing seems to fix it.
Did you ever figure out why you were seeing the same thing? I'd really appreciate an update as to how you fixed it (or if you fixed it at all.)
Thank you!