That you have a mac shouldn't make any difference. That you have curse shouldn't either - although perhaps you ought to see about getting decursed.
Make sure you install the addon while WoW is not running. Make sure that, if necessary, the "load out of date addons" button is checked in the addons panel at login (I don't think that's necessary with nui, but check it anyway). Make sure you're not using any other mods that do the same things nui does (bar mods, unit frames, etc.).
When you log in, you should get a notice in general chat that nui is loaded. I gather you're not getting that. I do wonder what, precisely, "not working" means.