You need to install OS X drivers for your cable. It looks like RT Systems only offers Windows drivers, so the best you can do is contact them and and ask if their cable manufacturer also supports OS X. If they use a common USB chipset like FTDI VCP or Prolific PL2303 you will be able to download OS X drivers from the manufacturer's website.
I think Robert confirmed that they use FTDI, but with a weird vendor code, which makes the vanilla FTDI drivers not identify it without a little legwork.
Kerry, to answer your question, I think the *easiest* think would be to get the OPC-478U, or the OPC-478 and a mac-friendly USB adapter like the Keyspan. I think it's *feasible* to get your RTSystems cable working, but it may be more trouble than you're looking for.