1: Extract the .zip file, and place the resulting folder into "~/Library/Application Support/Steam/SteamApps/Common/Portal 2"
2: Unzip this file and place the resulting application into your Applications folder, or wherever you want. [link]
3 (Optional): In Steam click the "Add A Game..." Button in the bottom left corner, then click "Add a Non-Steam Game". In the window that pops up find "Portal 2 - The Core" (The app you downloaded in step 2) wherever you put it, select it, and click "Add Selected Programs"
IF YOU DON'T DO THIS STEP THE MOD WILL NOT SHOW UP IN YOUR GAMES LIST ON STEAM.
4. You can now launch The Core by either opening the application downloaded in step 2, or normally through your library page in steam (If you completed step 3). If steam is not open when you open the app it will be opened for you.
1: Delete thecore folder from "~/Library/Application Support/Steam/SteamApps/Common/Portal 2"
2: Delete the application from step 2 of the installation
3: Right click on Portal 2: The Core in your steam Library page and select delete shortcut.
CosmicD: Feel free to repost this wherever you want, so that us mac users can more easily enjoy your wonderful mod.
PS this is the same solution that luvon posted on the thinkingwithportals.com thread, but with the launch options he described (minus the -dev, it was unnecessary) integrated into a shortcut (the app from step 2).