"Activation of Security Retail card just follows the website www.mcafee.com/activate & then it will ask you to enter the 25 characters long McAfee product key. Once you have entered the correct McAfee code & other details correctly on the activation screen then you can start the download and installation of your Antivirus Security Online."
Already Used Mcafee activation code?
In case the McAfee activation code is already used, and you wish to activate a McAfee antivirus on any other device with the help of the same card. Then you are required to go to mcafee.com/activate and then press the “Download Your Software” option presented at the bottom of the web page.