you can get just a null modem cable, or you can use the rs232 cable you have and purchase the null modem adapter as rolly noted. null modem is not an actual modem, it is just an adapter that reconfigures the wiring of the cable so it can communicate to a device.
there is a program out there that runs on a laptop, that will communicate with the 401, i have used it a few times, and it worked fairly well. I liked it better then the OMT.