cannot open on Nook, "This Nook cannot read this file. It may be an unsupported format" what can I do?

[nookbook]

I used ADE 4.0 and ebook transferred to computer and Nook - it opens on my computer but will not open in Nook.  Error message says "This Nook cannot read this file.  It may be an unsupported format."  What can I do to open it on my Nook?