All standard plugins shipped with op5 Monitor are installed in:
/opt/plugins
The macro you use to reach the plugins folder is:
$USER1$
The plugins you add to the system by your own must be placed in:
/opt/plugins/custom
And they will then be reached with the following macro/path:
$USER1$/custom
The reason for placing your own plugins in /opt/plugins/custom is because then they will not be touched by any upgrade from op5.