You have got the power supply plugged into the shield haven't you? In that case it should work when the switch is the the 'Ext' position.
On mine the other position is marked 'Xduino'.
You have got the power supply plugged into the shield haven't you? In that case it should work when the switch is the the 'Ext' position.
On mine the other position is marked 'Xduino'.