LEGO Marvel Super Heroes Steam Key Game Activation: 1. Open Steam 2. Click on the Games menu 3. Select Activate a Product on Steam 4. Enter the serial key and click Continue, then follow the on-screen instructions (Please enter the code manually or copy-paste). After that, the game will automatically be added to your library and can be downloaded immediately. Once the download is complete, the game can be played right away. The game will be available in your Library.