I have reviewed the MFJ website and I am still not sure of a solution. I believe you want to connect your computer to this 'Keyer/Reader' using the serial port on the device. I have no idea what this is, some sort of short-wave Morse code thingy? (sorry I'm clueless.)
The Terminal program on your computer is a terminal emulator, but it is not capable of communicating with serial ports.
I don't know if zTerm will do what you are looking for, but it is a terminal emulator that can talk to serial ports. If you can get zTerm to recognize the serial port, you can configure it using the settings you need to match the device.
http://homepage.mac.com/dalverson/zterm/
Sorry I can't be more help. It has been a while since I have used a serial port to communicate!
Andy