The plug-ins folder on Windows is located inside of the user directory.
%LOCALAPPDATA%\Plex Media Server\Plug-ins\
Using the Finder's "Go" menu, select Go To Folder... then enter the following in the dialog box that pops up:
~/Library/Application Support/Plex Media Server/Plug-ins
In general, the Plug-Ins folder for the various Linux versions of Plex Media Server will be found under:
$PLEX_HOME/Library/Application Support/Plex Media Server/Plug-ins
NAS Devices and Other Linux-Based Installs
Plug-ins files will be located in a
Plug-ins directory inside the main Plex Media Server data directory.
Related Page: Where is the Plex Media Server data directory located?