Hallo,
many things changed since the last beta, and this might be related to the versions collision. Please, send Yor Activity Log (there's an error number returned by the scanning service, probably 15 - the engine cannot find its 400.vps).
Thus, get rid of the old version first (after that, You must see "Install", not "Update", in the proceed-button in the installation window):
do (as root): rm -fr /Applications/avast*; rm -fr /Library/Receipts/avast*
and then, launch the avast.pkg installation package.
regards,
pc
