Be advised I am not a normal MAC user but I am an nStep'er they are my device, circuitboard/firmware and working with Jasem to get the INDI driver running a few years ago.
MAC should need nothing added to be able to see and be able to talk to the device as long as you version OSX supports USB CDC devices. It is not special like FTDI needing a low level driver.
Which version of the nStep? Small in a sleeve or box with buttons? With Wifi?
Does it show with an assigned port under port selector?
Do you see a /dev/tty.usbmodem* and /dev/cu.usbmodem* (on High Siera anyway).
Does it show up in 'About This Mac'->SystemReport->usb ?
You can try talking to it with something like Zterm (or other terminal app that can connect to a usb serial device).
Open ZTerm, select the device, hit a few ctrl-F's, should get an 'S' back for each hit.