CE_COPY_NOT_ALLOWED

Pls can someone help a complete novice.

I was given a Nook for my birthday yesterday.  I was also given a voucher for ebooks.com.  I followed all the instructions when purchasing the books, downloaded Digital Editions etc.  I can see the books in DE but when I try to drag and drop them to my Nook the error CE_COPY_NOT_ALLOWED appears.  I see from other discussions that this could be something to do with authorisations.  I have no idea how to check whether the log in for ebooks and DE are the same.  Can I start again?  Ideally I need a step by step guide as to what to do.