It's easy to see that I have the latest version of Calibre. But that's all that is easy for me. The book is an .AZW book. The Calibre for OSX plug in window shows a bunch of Conversion Input plugins, including AZW4 Input (1.0.0) by John Schember, which doesn't look like the right thing - but is closer than any other plugins in that section.
Telling Calibre to update plugins finds an empty window.
I downloaded tools_v5.3.1 from Apprentice Alf's link and saw some files are different from those in the directory where I stored them in the past, and copied the new ones over. But I knew that would be insufficient - there has to be some way to implement them. I tried running a couple of programs, but need to look up how to tell Mountain Lion to run the program not supported by Apple (I kept trying hitting control, command, option, etc while clicking it). But those aren't my goal - my goal is to get the plugin working again. The KindleBooks folder has a bunch of files in it's lib which I copied over to a similar folder in my Calibre files.
I didn't expect copying files would be sufficient, but I am not finding out what I'm supposed to do to make sure my plugins are updated and working.