Adobe digital edition 3

I have installed Adobe digital edition 3 (ADE) on my Mac computer.  However, when I click on the icon (looks like a book).  Nothing happened.  I tried to download book from public library to my nook.  The file did transferred however, Nook cannot read the library book file.  It gave a message as This Nook cannot read this file.  It may be an unsupported format.  I think that my ADE wasn't opened, that's why the nook cannot read the file.  If that is the case, why the ADE is not opened after installed it?  Thank you for helping!