It sounds like Adobe Digital Editions might require Mac OS X 10.7 now (just based on a search of other people having this problem). I do not currently have a Mac OS X 10.6 machine available to me to test this with though.
Is it possible for you to upgrade from 10.6 to 10.7?
Maybe someone from Adobe can update the documentation at Tech specs | Adobe Digital Editions Home
I looked to see if there was a software update to 10.7 and it said no software update available. They probably changed the requirements to 10.7 and it is not reflected in the digital editions requirements, thanks for your help anyway!
Apple does have an update to 10.7 (Mac OS X Lion - Wikipedia, the free encyclopedia). You might want to see if your Mac is eligible for the upgrade (some of the older Macs are not able to run 10.7) 10.7 might also have been the first upgrade for OS X that was made available through the App Store, you might want to check there as well!