If this error message appears, you’ll need to downgrade your version of Adobe Digital Editions.

1. Navigate to the Applications folder. To do this, click Finder (bottom left of dock), then click on “Applications” on the left-hand side.
2. Remove Adobe Digital Editions from the Applications folder. To do so, click Adobe Digital Editions in the list (highlight) and delete (move to trash), or right-click the application and move to Trash.
3. If you have OS X 10.7 “Lion” or newer, install Adobe Digital Editions 3.0 from this link.
4. If you’re using an older version of OS X (before 10.7), install Adobe Digital Editions 2.0 from this link.
5. Try downloading your book again and open with Digital Editions 3.0 or 2.0.
6. Enter your Adobe ID when prompted, and then you should see your book begin to download.