Try this. Right click on your ePub file, choose Open with and then choose ADE. This is in Windows. Not sure how it is in Mac.
On a Macintosh, open the Finder. Select an EPUB file. Choose File > Get Info. In the Open with section, select Adobe Digital Editions 2.0 from the application menu. Click the Change All... button. Now all EPUB files should open in ADE by default.
Thank you for your help. I found out from another person responding that in the "Get Info" and then I can change all the documents to open with that program. Thank you for responding so quickly.
Ciao for now,