If you are using calibre's "Send to device" functionality (which as you have Kindles you should be) then you *don't need* this plugin. The same functionality is built into calibre (unless you have disabled it on the Kindle plugin). See Preferences -> Plugins -> Device Interface plugins -> Kindle 2/3/4/Touch Device Interface -> Customise.
This plugin is only intended for users who are doing the "Save to Disk" approach to exporting books (presumably running Kindle reader software). And in that situation the apnx file is *not* automatically generated by calibre, so they would use this plugin to supplement the output with the apnx files.