Hi Captain Olly
I'm not with Laminar Research, just a fellow simmer in the UK, but I guess that installing X-Plane 11 related liveries for certain aircraft on an iMac, is just the same as you would do on a Windows PC.
It rather depends on which aircraft types you are referring to, but most civilian airliners for example, you will find a liveries folder inside the main aircraft folder it relates to. You need to look in File Manager for these.
There are a few light GA aircraft in X-Plane 11 that don't actually have a liveries folder, but for most larger aircraft, there is one. The following link shows an example of this for my Zibo Mod Boeing 737-800X :- http://questions.x-plane.com/?qa=blob&qa_blobid=9816844507715255580
Naturally X-Plane 11 ships with a number of aircraft liveries already included, but if you wish to download others, then a good place to start for airliners is:- https://liveryfinder.info/index.php.
For lighter GA aircraft types, you could try the LR's own website:- https://forums.x-plane.org/index.php?/files/category/2-aircraft-skins-liveries/
Hope this helps
Regards