1) Register on the EA website - https://www.ea.com/profile/register
2) Download and install EA "Origin": Origin client - http://www.origin.com/download
3) Run EA "origin" and enter the purchased key on the "Activation" tab.
4) Log in using the email address and password you provided when registering on the EA website. If you are not yet registered on this site, then create a new account.
5) Download the distribution.