I want to say the one labelled "PWR", but.. I'm not so sure.
They referred to a JP1, but there's no such label on the board.
I don't even know if I can trust their schematic (X1, X2, but what about M!-4?) at this point.
It's that bad. I could drop an F-Bomb.
If the shield is plugged in, powered, and with nothing parked on the 2-pin "PWR" header, is there any voltage at EXT_PWR?
Of course, you could log in over at
http://forums.adafruit.com/
rustle around, it may have been discussed already, or pose a question.
It's not as active as it is here, but it's worth a shot.