In order to activate a Blizzard game key, a Battle.net account is needed (you can create one here). After that, simply follow these steps:
- Go to Blizzard and sign in to your account.
- Click your email address from the upper right corner, and select Account Settings.
- On the Account settings page, on the menu on the left select Account Overview.
- Paste your game key in the designated area and click the Redeem Code button.