I can't open a library epub book, let alone put it on my Nook

Hello.  I have Windows 10 and Adobe Digital Editions 4.5.2.  I am trying to download epub books from my library and put it on my Nook to read.  When I download the file, I can see the book in Digital Editions, but it doesn't let me click on it and the File, Library, and Help are all greyed out.  I can't even click the X to close the window, I can only close it by going to Task Manager.  Please help.