Years ago, Max had a single "plugin.ini" in the root directory of the software. Adding a custom path in this ini would allow you to put plugins in a central location on your network rather than inside each installation itself. Now this "plugin.ini" is placed in each user's installation directory:
"C:\Users\*user*\AppData\Local\Autodesk\3dsMaxDesign\20XX - 64bit\ENU" and called "Plugin.UserSettings.ini
If you try creating a custom "plugin.ini" and placing it in the install directory like the good ol days, Max will ignore it.
If you want to set custom paths to plugin locations, it is done by editing this file, but beware, the first time you render with max under that user, max will re-write this file to it's default. Once this initial rendering, right after install, is done, then you can configure this ini as desired.