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You go to the PSN Store on your PS3 and then to "Redeem Codes" and enter the code there.
You can check more on the Activision support site below and raise a support call if you have problems.
http://support.activision.com/articles/en_US/FAQ/Purchasing-ELITE-Premium-Member ship/?q=ELITE+Premium+Membership+PS3&l=en_US&c=Game_Title%3ACall_of_Duty_Modern_ Warfare_3&fs=Search&pn=1