To access your license key, you can navigate to and login with your username and password. Additionally, the license key was also emailed to you when you downloaded the theme, in case you did not create an account.

The license key can be entered by navigating to Appearance > Theme Options > License when logged in to your website.

Here is a more detailed article, showing you how to access you license key and activate with more visuals and a more in-depth walkthrough: