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installer -verbose -pkg maxwell_maya_osx_23.70.41.pkg -target /
The installer needs superuser privileges, so you may need to add sudo before the command.
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On Windows and Linux, the plug-in depends on the MAXWELL2MAXWELL3_ROOT environment variable being set correctly. If this variable is missing or wrong, the plug-in will be unable to start the renderer or the standalone material editor. The Maxwell installer sets this variable on Windows, but you have to update it if you ever move Maxwell to a different directory. See here for more details.
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The plug-in doesn't require a Maxwell license in order to run. The only restriction in the absence of a valid license is that the horizontal resolution of the FIRE viewport is limited to 720 pixelsload, but scene export, rendering and FIRE are disabled if a valid license cannot be acquired.