First of all which tty? It should be /dev/tty.usbmodem1 for USB with any speed (you will not see boot log here) or /dev/tty.usbserial-XXX on SERIAL5 (see Wiring page) to get main nsh console with boot log.

I like `cu` more than `screen`, because it allows to scroll:

    sudo cu -l /dev/tty.usbmodem1

or

    sudo cu -l /dev/tty.usbserial-XXX -s 57600