Saying you are moving this to Adobe Digital Editions is not helpful. Telling me where the answer is would have been more helpful. I've searched on the Adobe web site multiple times in multiple ways and can't find an answer.
Saying you are moving this to Adobe Digital Editions is not helpful. Moving it is helpful as it gives a greater chance that it will get read by somebody who knows about ADE.
Sounds as if your install has almost worked, but the file association bit hasn't.
Try running ADE from the provided icon. Once in ADE do a file open (ctrl-O, or probably cmd-O on a Mac?) and see if it will open the .epub file.
Or you should be able to drag-drop a .epun from Finder onto a running ADE.
If those do work, you need to use standard Mac methods to force the file assocation. ( http://osxdaily.com/2009/10/25/change-file-associations-in-mac-os-x/ )
If that's the case, I don't know why file association didn't work on install. I know people have had similar issues with a Windows install as well.