Hi, Susan.
All you need to do is:
1. Download Kindle for PC (K4PC) from this page:
http://www.amazon.com/gp/feature.htm...Id=1000493771#
2. Buy your book, telling it to download to your Kindle for PC.
3. On your computer, look in your "My Documents" folder for a new directory, called
"My Kindle Content."
This is where K4PC puts the books you buy for it. (They'll have a .AZW, AZW1, or TPZ extension. Ignore the .mbp files.)
Now, you can drag that file (book) into calibre.
To read it on your Nook, you must convert it to ePub, and if the book has DRM, you will need to strip the DRM first. (If the book won't open in calibre's reader, it has DRM).
I can't give instructions on how to to strip it, but googling should lead you to a package of Tools, which include a mobi K4PC de-drm plugin you can install in calibre ... follow the easy instructions to use.