Using ADE on PC; downloaded ebook and it shows up under "My Digital Editions" folder under Documents, but ADE won't add it; I get error code "System.IO.FileNotFoundException"; please help!

I searched for all possible problems.  I have only used one computer so I'm not trying to access a file downloaded to another computer.  I added Adobe Digital Editions as an excepted program under windows firewall.  What else could be the problem?