To use the purchased license, sign in or create a new JetBrains Account using the "Sign in/sign up to JetBrains Account" options presented at the top of the screen.
Once in your JetBrains Account, the license page will present you with the options to download an activation code for offline use or generate a legacy license key to use previous versions of the software. You can now use software by logging into it with your account.
Please note that Account login is the preferred method of signing into your software. You don't need to download an activation code unless your machine is restricted from accessing the Internet.