Cannot manipulate pluginkit database

I am trying to use the pluginkit command to handle plugins (specifically FxPlug3), e.g.

pluginkit -a /path/to/bundle/with/myplugin.pluginkit

Every time I try to add or remove my plugin I get a message of the type

add: unable to save changes: (null)

and in my console log

13/03/2015 22:05:50.537 pkd[927]: error writing database changes: (null)

I am looking for the pluginkit database so perhaps I can look for plugin specific data; according to the man page for pkd I can run pkd -W to learn its location. However pkd doesn't seem to be in my path and I can't find it.

I am using OSX 10.9.5