"master unlock button" ? could you describe the location?
Factory default for the SE&SS/SKS is if the fob is in the antenna range and touch the handle of: driver's door unlocks driver's door only, hatch or passenger's door unlocks all doors.
(You can reset this to unlock all doors when you approach/touch the handle, or to only unlock the door you are next to only.)
The black button on the outside of the door is to lock all of the doors with the Smart keyfob.
If you use the remote keyless entry, one unlock press will unlock the driver's door only, two unlock presses will unlock all doors. (If you don't open any doors within about 45 seconds, the Prius will relock itself.) One button press to lock all doors.
The rocker power door locks button on the driver's and passenger's door should lock or unlock all doors.
The individual door lock/unlock button should work on the front doors, if they don't on the rear then you might have the child safety locks on.
I usually suggest locking the Prius using the remote keyless entry, or the black button with Smart-equipped cars, so that you know that the doors are all closed (and with Smart that you haven't left a key inside). With an open door, it's possible to drain your 12v battery overnight, and it's very easy to accidentially leave a door open...