Found this at their site:
www.mezzogmr.com
G4 and OS9 users: Mezzo 3.6.2 (or older) is not compatible with either of these newer items. We are working on an update that should be available soon. Look for 3.6.3 or newer to be on our web site very soon.
I hope they make this upgrade free of charge because I do not want to pay another 500 bucks so I can use it with OS 9.