In order to activate your product follow these instructions:

Download the Avast Antivirus application from here.
Install the application.
Right-click the Avast  icon in your system tray and select Subscription information.
At the top of the Subscription screen, click Insert activation code.
Type or paste the previously copied activation code into the box and click Activate.

After the activation code is accepted, the text Your products were activated appears. Information about your subscription such as the expiry date and number of days remaining is also visible in the Antivirus section of the Subscription screen.