Thanks. That fixed it.
I was used to getting prompted for automatic updates, so I thought my app was up to date.
Oddly, I got the version 0.8.48 from checking 'get info' on the app in the Finder.
But the version at the bottom of calibre's preferences pane shower 0.8.49
I manually downloaded and installed a new version and they both show 0.8.49,
and the plugin works.