After you have redeemed your Battle-Bot pet in-game, every day you should go back to the Mountain Dew Game Fuel website and click again on the Battle-Bot pet image (bottom-right).  It will forward you to the Account website.  When you login each day you will be able to choose red or blue fuel containers.  Afterwards, login into the game and your Battle-Bot’s cylinders will fill up.  This will allow your Battle-Bot to power up to attack other Battle-Bots.