Using a serial to usb adapter with MacOS X can initially be a challenge. You must first determine the interface that is being used. So issue this command to view the available interfaces;
ls -la /dev/tty* -crt
Locate and use the interface that appears to be usbserial/usbmodem with this command;
screen /dev/tty.usbserial* 9600
screen /dev/tty.usbserial 9600