1.Go to http://store.steampowered.com/about/

2.Click "Install Steam"

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3.Once the client is installed and has started, log in with your username / password (create one if needed).

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4.Once logged-in click "+ Add a game" in the bottom left corner (Or use the "Games" tab in the main menu).

5.Select "Activate a game on Steam..."

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6.A screen will ask you to accept the "Steam subscriber agreement". Click accept.

7.A window will prompt you to enter the code you purchased.

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​8. Steam will authenticate your code after a few minutes.

9. The game is now visible in your list.

10. Click it to start the download.