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Download and run the Steam client on your computer.
Log in and then select ADD A GAME in the bottom left corner of the screen.
Select the option Activate a Product on Steam from the drop down list.
Read the information displayed and continue.
Read and accept the terms.
Type in the specified field, your key from Gamehag (you can copy it from your inventory).
A notification will be displayed informing you that the game has been added to your library.
The game is yours. You can install it now or later.