@mwyoung - welcome to MobileRead.
These sorts of errors happen when people forget to restart calibre after installing plugins. Make sure you restart it, then try the install again, restart calibre again and then try the plugin.
If you still have issues it could be your calibre installation has an issue. Uninstall calibre (you will not lose your library), reinstall it, install the plugin and restart.
Any time you install or update a plugin you must restart calibre - stick to that and you shouldn't have any issues like the above.
@Kovid - I haven't tried calibre's plugin load option/plugin updater option lately, but if you haven't already done so already I wonder if we should fiddle things so users are given the "Restart now" dialog like they get if they make changes elsewhere in Preferences?