Calibre tends to associate itself and its viewer with every conceivable format for what might possibly, maybe be a book. .txt files, for instance. Which can be annoying when you want to edit a config file, say...
Go to Preferences|Interface -- in the lower right corner is a list of formats that Calibre's viewer associates with. Uncheck any that you would like to use another viewer with.
Then, however Macs do it, associate the format with the ADE viewer (system-wide). I expect it has something to do with viewing a file's properties, but I'm not a Mac guy.
Now, whenever you try to view it in Calibre, you should get the ADE viewer. I think.
This all assumes you are just trying to find a way to read your DRM'd book, not disinfect it.