Click on the Arrow head in the system tray, then right click on the Avast logo
From the choices, click on Open avast! User interface
From the left menu, click on Settings
From the left menu, click on Subscription
Locate and click on Insert license file
Browse to the folder where your file was downloaded from your email. (most likely the downloads folder) Click it once to select it and then click Open
You should now see a message that your subscription is active and how many days are remaining.
If you have any problems please create a ticket and someone will be in touch shortly.