The PL2000 just needs a serial port, running their proprietary DOS program. It's not like the earlier versions of ProLight mills that required an interface card in the PC.
It sounds like you may not have configured the serial port correctly.
I have a PL2000 running off a PC, and I can talk to it. I can't remember my port setting though, and have forgotten how to read them back from a DOS prompt. I'd be happy to send them to you if someone can tell me how to extract them from DOS.
Jerry