Hi, I'm new here AND I'm a MAC User. I downloaded my McGimp at Partha's Place and followed the link there and then provided to the NIK collection. It was for free at that time. I assume you have that .DMG file, if not it should be available somewhere on the internet in an archive. (Or ask me, I saved mine) When clicked it will install all those beautiful filters as separate apps including an app to remove them (but why would you?). Once installed, open your McGimp, open an image and go to: Filters and scroll down to the NIK Collection and take your pick, play around and if you're happy with it go to the right bottom corner and click to save. The app will close and with a warning that your image will be reloaded it will show up, including the changes you made.
I hope this helps, I'm Dutch so please be patient with me!
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Paula Bennink-van Beekvelt
Mac-user, GIMP-User