Not sure how this works on the Mac but when you extracted the zip file did you then copy the folder inside that folder into the addon folder ?
I am sure its set up is the same as nUI regular but if you used curse to install that its possible that you are not aware of the folder structure inside.
Double check the addon folder structure and make sure you see this folder pattern.
Interface/AddOns/nUI/nUI.toc
Interface/AddOns/nUI/Main/nUI.lua
If you don't see that folder structure then something went wrong with the extraction/installation.
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