In order to link a gamertag or PSN ID to your Origin account, you need to do this:
1. Log in to your Origin account
2. Press the cogwheel in the top right corner and choose "add a friend" (if you are on the pre-release version, click on Friends in the top left corner and choose "add a friend...")
3. The pop-up will give you the option to connect to Facebook, XBOX Live or Playstation Network. Choose your preferred option
4. Fill in your log-in details and follow the steps on screen
You're good to go. If you are unable to connect, it could be a temporary issue with the servers. Try again later and if the issue persists for more than 24 hours, contact EA support.
*Bangs head on desk"
The cogwheel is the little round icon that looks like a wheel, or a gear.
