Mad Moxxi's Underdome Riot is an addon pack for Borderlands that includes a new cooperative arena game mode and a stash to store your gear.Are you god's gift to gun fights? Think you're the best? Wanna prove it? Then help us celebrate the grand opening of Marcus Bank (a subsidiary of Marcus Corp) by killing hundreds and hundreds of people in Mad Moxxi's Underdome Riot, the only competitive arena around where your next of kin can be assured that you're coming back famous . . . or not at all. (All proceeds are kept by us) Multiplayer options may have changed, please see the main title for more information

Go to: http://store.steampowered.com/ and download STEAM client Click "Install Steam" (from the upper right corner) Install and start application, login with your Account name and Password (create one if you don't have). Please follow these instructions to activate a new retail purchase on Steam: Launch Steam and log into your Steam account. Click the Games Menu. Choose Activate a Product on Steam... Follow the onscreen instructions to complete the process. After successful code verification go to the "MY GAMES" tab and start downloading.