PLEASE READ MY PREVIOUS POSTS CAREFULLY!
The fact that SS6 on Mac OS connects to a Celestron (NexStar) SE and other Celestron mounts is irrelevant, The CEM 40 uses a different protocol than Celestron. Selecting Celestron SE mount in SS6 Telescope Setup... results in the same error message when clicking Connect! "Connection Failure The scope is not responding . . ." Also, according to Celestron the USB B port is intended for use with their CPWI software which is Windows ONLY developed by PlaneWave. The mini-USB on the hand controller is entirely different but I don't know what modifications have been made to the protocol/firmware/hand controller to allow "direct" USB connection.
But whatever the case, Celestron uses a different protocol for USB communication than the iOptron CEM40.
The virtual COM port is part of the firmware on the CEM40. It replaces the serial-USB connection used on other iOptron mounts.
This port is addressed through the FTDI USB to RS232 VCP driver that must be installed on the Mac.
The protocol for controlling the CEM40 being used by SS6 apparently is not utilizing the driver or virtual COM port correctly. Several other telescope control software apps have worked this out so the issue is resolvable.