I just removed the Prolific drivers Serialio makes available on their site (http://www.serialio.com/products/adaptors/usb_serial.htm) and tried installing this driver but I am not having any luck using it with Zterm. It does not show up as a device option for my connection.
Any ideas on where to start troubleshooting?
It might just be the device naming. If you know the embedded serial number of the adapter (ref. System Profiler -> Hardware -> USB -> 'Serial Number:'), then the device will be named:
where abcdef is your serial number. If your device doesn't have a serial number, the driver uses the USB port address, a mostly meaningless number that is fixed for a given USB port.
The following will show you where it's turning up (in Terminal, after plugging in the device):
$ ls -l /dev/tty.PL2303-*