You can get the free updates here, install as a regular package.
http://free.avg.com/download-update-7#tba1
You also need to run the following commands to get the update to work:
sudo chmod 775 /opt/grisoft/avg7/bin/avgupdate
cd /opt/grisoft/avg7/var/
sudo chmod 777 run
sudo chmod 777 update/log
I also needed to manage groups and add everyone to the AVG group, logout and login again to pickup the new virus defintion files.