The Emax requires a specific pin assignment, so a custom cable should be made. Common retail RS232/RS422 cables won't work with the Emax.
Note that normal commercial USB/RS422 adapters won't work neither. The USB to RS422 adapter should be a custom-built one, as explained in the EmuSer DIY project. Did you buy an EmuSer or a standard retail USB/RS422 adapter ?
See the EmuSer documents for both the cable and the adapter specifications on http://www.emxp.net