1.) To activate your license, first go to My Account, you will find all active subscriptions here. Copy the purchase key.

2a.) Login to WordPress dashboard. You can run the Autoconfig in Setup Wizard, in the first step you can set the purchase key.

2b.) Alternatively you can go to settings to set your purchase key. You will find it in Swift Performance Settings » General » Purchase Key

3.) Once you set up purchase key, your license is activated. You can maintenance your sites and subscriptions in My Account » Sites