Additional plug-in paths are loaded from two locations:
The User Profile Plugin.Usersettings.ini file can be used to augment the 3ds Max plug-in paths on a per-user basis. These user paths are exposed to the user in the UI when opening the Configure System Paths dialog and clicking on the 3rd Party Plugins tab. This UI feature shows only those paths mentioned in the user's own Plugin.Usersettings.ini file, if it exists, preventing access to system-wide plug-ins in this manner.
The plugin.ini file under 3dsmax\[locale] can be used to augment system-wide plug-in paths, but requires admin privileges to write to.
There is no definitive method for programmatically locating an individual plugin.ini file in the User Profile directories.