Click on Maintenance, than Registration for online registering.
1) Open Avast.
2) Click on "Maintenance" then "Registration".
3) Fill out the form for your license key and click "Submit".
When the time is about to expire you will receive a popup notice to register it again.
Note that you will also need to do steps above if you ever need to reinstall the program.
If it fails (or you're in an offline computer), try offline registration.
Fill the form here ( http://www.avast.com/registration-free-antivirus.php
) and enter (copy & paste) the serial number you've recieved by email.
Inserting the license key into: avast! Free Antivirus