sudo launchctl remove com.apple.xserve.serial-ports

screen /dev/cu.serial <port_speed>

Replace the <port_speed> with the port speed supported by the device you are trying to control. This should allow you to connect to the device and control it.

When you are done, you can reset the Terminal by performing these key strokes

Type Ctrl-A and press Enter, then type Ctrl-\ and press Enter.