Downloaded book shows up in my downloads, but not in ADE so that I can copy it to my Nook. Why? How to fix?
If you are not fulfilling the book through Adobe Digital Editions you have to add the book to Adobe Digital Editions through their menu option (Add to library) for doing so.
Adobe Digital Editions does not scan your entire computer looking for files that it knows how to open, it needs to be explicitly told about EPUB or PDF files that you would like it to know about.