l'm using 5v for the arduino board and 5v fro the motor shield
The External power supply for Arduino Unos needs to be 7-12V.
From this spechttp://arduino.cc/en/Main/ArduinoBoardUno
Input Voltage (recommended) 7-12V
The board can operate on an external supply of 6 to 20 volts. If supplied with less than 7V, however, the 5V pin may supply less than five volts and the board may be unstable. If using more than 12V, the voltage regulator may overheat and damage the board. The recommended range is 7 to 12 volts.