These instructions will help you unlock your Google Sheets add-on with your activation key. Please follow the steps below (by example of Power Tools) to activate your subscription.
How to activate the add-on
Step 1: Start the tool
Go to Extensions (or Add-ons if you have an old version of the Google Sheets menu) > Power Tools > Start:
Tip. Or, you can access the activation by clicking Extensions > Power Tools > Activate my subscription right away:

Step 2: Open the Activation window
Click on the key icon at the bottom of the add-on sidebar. Copy and paste your key from the registration email into a special field in the tool and click Activate my subscription:
Step 3: See your subscription details
As soon as the subscription is activated, you will see all the related info in one place upon clicking the same key icon at the bottom of the add-on window: