We are currently investigating an issue where liquivid refuses to open video files. This affects Mac OS X only.
WORKAROUND for those affected:
Instead of starting liquivid from your library in the Steam client, start the app directly through Finder: In the Steam client right-click liquivid -> Properties... -> Local Files -> Browse Local Files... -> double-click the liquivid app to start it).
Our current guess is that this is related to a recent update or change in either Steam or Mac OS X itself. We'll keep you updated when we have any news.