The avast! beta is free. However it still contains a license which can expire. During the beta stage we set it about two months since the build. Solution is to install newer beta build which has prolonged the license duration.

You can always find the latest version in the (continually updated) first post of this thread:
http://forum.avast.com/index.php?topic=78646.0

Currently it is http://public.avast.com/dev/mac-beta1/avast-29138.dmg

Regards,
Mity