Valve Software develops and manages "Steam," a digital distribution platform. Users purchase games through "Steam," which add to a library. Players install or delete games from their computers using "Steam." Many games now offer DLC, or Downloadable Content, which adds more content to a particular game. This DLC can be purchased from "Steam" itself or from other sources. If you purchase the DLC from other sources, you manually add it to your "Steam" game.
Locate your DLC key. If you purchased the key separately, from a retail or online store, your code may be printed on a card or on a receipt, or in an e-mail. DLC keys are a series of letters and numbers that you can input into "Steam" in order to gain access to more content in your games. For example, purchase the "Elite Units of the East" DLC for "Empire: Total War." Keys for any product activated on "Steam" must follow key guidelines established for each game. Keys not in these formats cannot be activated on "Steam."
Click "ADD A GAME" located in the lower left corner of the "Steam" window. A small menu opens above this button.
Click "Activate a Product on Steam." Click "Next." Read the agreement and click "I AGREE."
Type in your product code in the "Product Code" box as it appears on your receipt, card or e-mail. If you received the code online, such as in an e-mail, copy the code from its source and paste into the box.
Click "Next," which activates your DLC. The DLC will be automatically downloaded to the game in your games library, or automatically installed with the game if you have not yet installed the game.
If you purchased the DLC through "Steam," it will automatically be installed into your game. You do not need to manually install it.