Calibre cannot locate books on the Kindle Fire if they were downloaded from the Kindle Store, although it can locate books that have been sideloaded to recognizable locations. (Like calibre does.)
If you wish to add your Kindle books to calibre, install the Kindle 4 PC app, and tell calibre to add the books from there.
Remember, the books will be locked with DRM, so they will be unreadable. If you know the drill,
, if not...
MobileRead has a policy about not giving guides in DRM circumvention, but we are allowed to say this much -- google "apprentice alf" for help.