you have to learn how to send ASCII string out of your PLC.
download ZPL manual and see commands. it's straight forward.
you can also:
install this printer to your laptop and load drivers and software
and try to print sample label. once it works, use crossover cable
to connect to another COM port. use hyperterminal to capture
stream that your PC is sending to laptop. paste this into your PLC
and try to send it out using PLC.