Instructions
Launch your Steam client and sign in. Click on the "Library" tab at the top of the client window. A list of all the games you have purchased through Steam displays.
Select a game title in the left sidebar. An info page for the game appears in the main panel. Click the "Store Page" link in the right sidebar of the game info page.
Scroll down the store page until you see the field that reads "Buy this game as a gift for a friend." Look at the logo above the "Add to Cart" button. If you see a small image of the Apple logo and the Windows logo with the phrase "SteamPlay," then the game has been made Macintosh compatible. If you see only a Windows logo, then you will not be able to install that game on your Mac.
Return to the Library page. Select the game again from the left sidebar. Click the "Install" button below the game title within the main panel. A pop-up window appears informing you of how much disk space is required for the game and how much disk space your computer has available. Click the "Next" button. Click "Finish." Your game installs to your computer.