HELP! I'm unable to download/transfer ebooks from library to my Nook.

I'm unable to transfer library ebooks from my computer to my Nook.  Does anyone else have this problem?  [It is most annoying.  My public library is set up for Kindle, though they assure me that downloading to Nook should be possible.  NOT SO!]  computer uses windows 7.

Thank you,

Dr. Barb