Which error message you get exactly?
To get a book onto the Kobo, I found that I have to first save the ascm file and then double-click the file to load into ADE. Then, when I drag the book from my computer to the kobo, the book loads copies properly to the Kobo. When I opened the ascm file directly, it loaded into ADE just fine, but forget getting a readable copy on the Kobo. I hope this helps.
Connect you reader with the Device with Digital Edition. Its should appear the Reader in Digital Edition. Now you can move the File to the Reader.