I talked to Customer Support. Here's the procedure:
1) Install your first copy to your PC, and activate the license.
2) From your keyboard, hold your Window key and R together to bring up the Run pop up window
3) Type iexplore home.mcafee.com, and hit OK to bring up the McAfee home page
4) Select My Account tab, then Redeem your Retail Card, then enter the 25 digit from the card to extend to another year. You may have to log in.
5) Repeat step 4 if you have more cards (up to 5 in total)
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