I think there are two ways, one in the settings within the menu (check manual)...the easier, press and hold both lock and unlock on the key at the same time. Do so for like 3 seconds until you here a beep. On the info screen it will show the car with all 4 doors opened.
Already been discussed please see link below. http://priuschat.com/forums/prius-c-main-forum/108035-door-unlock.html