The game is found on the Steam application. Click here to locate that page.
Scroll down on the page until you see the "Install/Play Game" logo. 
Once you clicked the button, a download screen will appear. Choose the folder you'd like the game installed and allow the install to take place.
Once you have installed the game, you can access the game from the "Steam" application under your "Library" of games. Locate the game on the list, click on the game and hit "Play"