...
The plug-in can be installed silently by passing the /S flag on the command line:
maxwell_maya_win_2.67.234.exe /S
In this mode, the installer will use the autodetected directories and will not ask any questions.
...
installer -verbose -pkg maxwell_maya_osx_2.67.234.pkg -target /
The installer needs superuser privileges, so you may need to add sudo before the command.
...