I'm not familiar with the Fantom, but I know that some drivers are extremely picky about the procedure used for installing them.
To give my Yamaha keyboards as an example, when installing the Yamaha USB-MIDI driver I must be sure to connect my keyboards to the computer and turn them on before trying to install the driver, because the driver needs to be able to detect and identify the keyboards during installation; but when installing the Steinberg USB driver I must be sure to disconnect my keyboards from the computer and turn off any extraneous USB peripherals before trying to install the driver, because apparently if the driver detects any USB devices during installation they can mess up the installation.
So be sure to follow the installation instructions for the driver to the letter. Also, if you've already tried to install the driver but were unsuccessful, you might need to uninstall the driver before trying to reinstall it.
I know Roland has an driver for Mac, but it also appears to have a Profile file for Mainstage that needs to be downloaded. I don't use Mainstage or have a Fantom I'm just looking that Roland online manuals.