If the product has been purchased digitally, the registration code should be found in the email that was used to receive the digital data. Also look in the SPAM folders as well or Trash, if you didn't clear it.
Most of the products have a menu that can be accessed after the app has been started. You can go to Help > Register or Help > Activate to enter the serial number that you have to activate it. Additionally, look in the installation folder as well because there could be other executable files that act as registration methods. For example, in some apps you'll find a Register.exe app as well.
The ReadMe files usually found in the installation directory will also provide the necessary steps that you have to follow in order to successfully register the product.