This is how I do it.
check what device entry you have for your usb adapter, something like /dev/tty.PL2303
then do
screen /dev/tty.PL2303 38400
and this will launch a screen session on that device at speed of 38400.
to exit screen type
ctrl-a
then
d
this will bring you back to your shell, then kill screen with
killall SCREEN
if that doesn't work, try a different speed like 9600
to learn more about screen http://linuxgazette.net/147/appaiah.html