Have never done this but some similar project is in mind.
Try to first connect the GSM modem without the PLC directly to your PC. I think you must have done that. If that is working fine then see the PLC documentation on connecting digital modules and the software instructions for the interfacing part. HOWEVER remember the protocol of communication is ALREADY in the PC, for the GSM to work on the PLC you have to actually make the PLC follow that protocol, which might not be easy, so I suggest you try using a microcontroller with the PLC or just the microcontroller.
And please give a more explination of what exactly you are trying or have tried. There are different ways to do things
