Installation of Cedrus RB-740 response box

This is regarding the installation of the RB-740 response box. We have followed the instructions which came with the box and successfully added SR- box to the e-prime setup. E-prime is then showing the runtime error: 10051, asking to install two drivers: PST Port I/O driver and PST Serial Response box driver, which I am unable to find. Could you please guide which step I am missing or where can I find these drivers.