Product keys are used by Norton software products (as well as many others) to verify the legitimacy of the software being installed. Essentially, a product key proves that a user has paid for the license to use the software, effectively combating software piracy. Many Norton products can be installed without the use of a product key; however, after a predetermined trial period, the user will need to either buy a product key or enter a legitimate one in order to keep using the software.
Open your Norton product.
Find the option labelled "Subscribe Now." It's usually a button located in the main window of the program. If you don't see it, browse the menus of the program listed at the top. Typically, anything relating to activation is listed under the "Help" menu.
Select "Enter product key" or the option aptly named, otherwise the program will take you to a page where you'll be asked to pay for the software.
Enter your product key in the spaces provided and click "OK" when you're finished. You must be connected to the Internet so that the program can verify the legitimacy of your product key.