In Battlefield 3, players step into the role of the elite U.S. Marines. As the first boots on the ground, players will experience heart-pounding missions across diverse locations including Paris, Tehran and New York. As a U.S. Marine in the field, periods of tension and anticipation are punctuated by moments of complete chaos. As bullets whiz by, as walls crumble, as explosions force players to the ground, the battlefield feels more alive and interactive than ever before.
Go to: http://www.origin.com/us/about
Click "Download Origin" (from the upper left corner)
Login into downloaded application
Install and start application, login with your Account name and Password
(create one here: http://store.origin.com/ if you don't have).
Redeem Product Code
From the "Origin" drop-down menu (upper right corner) choose "Redeem Product Code...",
enter your code and click "Next".
Download your Game
After successful code verification go to the "MY GAMES" tab and start downloading.