If your Keyspan adapter is not showing up in the output device/port list in the cutter dialogue, it means that the computer is not recognizing it. To find the driver go to the manufacturers website of your Keyspan adapter and download the driver then install it.
Here you can download the driver for the most common Keyspan: Triplite
The prolific drivers do not work well with newer Mac OS X systems. We recommend that you purchase a USB serial adapter that uses FTDI for the driver instead.