ZTerm is not necessary any more:
Serial Communication in OSX Terminal
ls /dev/tty.*
to get a list of all of your serial ports. Pick the one that you want to open.
/dev/tty.BTRS232 /dev/tty.Tigoe6630-AppleAgent-1
/dev/tty.Bluetooth-Modem /dev/tty.Tigoe6630-Dial-UpNetwor-2
/dev/tty.KeySerial1 /dev/tty.modem
Knowing the serial port, you can just type screen portname datarate:
screen /dev/tty.Keyserial1 9600