What I usually do is extract it into GameData, and then look to see if my GameData now has an innner GameData underneath it. While some seem to leave the GameData off and expect you to extract it into gamedata, like this: About half seem to include the GameDarta in the path like this: Unfortunately there isn't much standardization between mod writers about how to package the folder paths in their ZIPs and RARs. (I don't remember if Macs come with a program by default that does RAR and ZIP but if not there should be free versions downloadable.) To install a mod, Unpack the RAR or ZIP file for the mod such that it ends up inside the GameData folder.

Steamapps -> common -> Kerbal Space Program -> GameData

Once you find that Steam libary folder the rest of the path is the same as for Windows from there on down: This post will help you find where the folder is: Supposedly they work the same as on Windows, but you just have to look in a different location for the KSP folder as the Mac version of Steam (I assume you bought KSP from Steam rather than directly from SQUAD if you're posting here) isn't in the same place.

Kerbal space program free mac