Install Digital Edition on your Mac: Herunterladen | Adobe Digital Editions
Start it and go to Help > Authorize Computer.
Enter your Adobe ID and your Passwort to authorize.
First if you have authorize the device successfully should open the eBook over file > add to Library.
Thank you for your help. I actually managed to open the eBook with Adobe Digital Edition. I tried so many things, that I do not remember how I managed to ope the eBook. Your instructions to open the eBook sound very familiar to what I did. Thank you for taking the time to reply to my question!
You are welcome.