1. In order to activate your The Elder Scrolls Online product, you will need to log into your Elder Scrolls Online account. ( You can create one here )
2. After you successfully logged in, You should be located on the main page of your Elder Scrolls Online account. There you will find a section called "Redeem Code"
3. Once you press it a "Redeem Code" box will appear. There you will need to enter your product key.
Please bear in mind that Upgrade DLC and Tokens can only be activated once you have The base game of The Elder Scrolls Online.