I've figured out the problem. For anyone else who might be struggling to connect their Mac to a Synscan V5 handset, you need the driver for the prolific USB to Serial chipset, which you can get here:

www.prolific.com.tw/US/ShowProduct.aspx?p_id=229&pcid=41

Despite having a USB output on the handset, there still is a usb to serial chip in there somewhere.

My MacBook Air can now control the mount through the handset.

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