which is the right location between var/lib/ or usr/lib

I use ubuntu 14.10, and I need to install plugins for adding library agent.

1. copy plug ins (xxx.bundle) to /var/lib/plexmediaserver/Library/Application Support/Plex Media Server/Plug-ins/
   first of all, I'm not sure this is right location because it's empty. There is no other plugins in the directory.
   result: nothing change.

2. copy plug ins (xxx.bundle) to /usr/lib/plexmediaserver/Resources/Plug-ins??
   actually, there is a Plug-ins-f38ac80 folder instead of Plug-ins folder.
   and the inside folder has bunch of xxxx.bundle files.
   but nothing works either.

I don't know where is the right location to put plugins so I tried both location, and seems nothing work.
I searched forum and FAQ the right location for the plug-ins folder would start with $PLEX_HOME/Library/xxxx
so I tried cd $PLEX_HOME/Library/ there is empty.

where is location should I put the plug-ins files.