Using Mods on a Mac is easy now that I figured out the location. Follow the directions included with each mod you want to use but if/when they reference the "Data", "Mods" or "Public" folders, on a Mac they are located at the path:
"Macintosh HD / Users / yourusername / Library / Application Support / Steam / SteamApps / common / Divinity - Original Sin / Divinity - Original Sin / Contents"
If you'll notice, part of that path contains "Divinity - Original Sin" twice. This is because the second one is an application container. You need to open it within Finder to access the rest of the path.
To do this, open Finder, click "Go" and then click "Go to Folder". In the popup box paste the following "~/Library/Application Support/Steam/SteamApps/common/Divinity - Original Sin/Divinity - Original Sin/Contents".
Click "Go" in the popup box. The path you want should now open.
Under the "Contents" folder is where your "Data" folder is located. This folder contains the "Mods" and "Public" folders that are referenced in some mods. If the folders do not exist under "Data", create them when you copy over a mod.
This should be all you need to use mods on the Mac.
Good Luck!
