To install it, I'm not sure, I use Windows. I only know that Custom Scenerys should be unziped in the "style/themed" directory of your RCT3 game. To get CS and cfr, I suggest you to search on google, because most of the RCT3 CS websites closed, including Vodhin's community, Das Matze's RCT3 Island, and rct-something.de, which was repertoring all custom scenery's or almost.
If you want to search, here are a couple of sets you should REALLY get :
Most of Das Matze's, including fences, office day, swizz it!, utilities (contains excellent footers for custom supports) etc.
ATH Catwalks, for catwalks on the side of your lifts
Jcat's Steelworks - supports, not very customizable but good starters
Moby's Steel jungle V.1 - custom supports, customizable but complicated
big burger's Timbers, custom wooden supports
etc.
As I said, I can't help for mac installation, I suggest you to wait for a reply or search on google or a video tutorial on youtube. Good luck !