Since you are in the HD/+ forum...
Check the permissions in your profile. Sometimes a download comes up with a "not visible in this profile" condition.
Second, in the library, use the menu to view archived items, sometimes an item gets flagged as archived.
What Wulfraed wrote should work.
Another option - download Nook for PC. Then download it to Nook for PC.
Then in Nook for PC archive and unarchive. This will put it on Nook.