Okay, take a look at your addon folder, wherever it is on the Mac, and see what the folder name is for nUI. It should be just nUI. If it is anything else then somehow it didn't install right. If it has a bunch of numbers etc like a version number then go into that folder and you should see a nUI folder .. that is the one that needs to be in the addon folder.
To visually see what I mean .. this is what the path to the toc should be for nUI.
AddOns/nUI/nUI.toc
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