How to get 3v3 pins on Arduino Mega Sensor shield v2.0?
They are all 5V as the Mega2560 uses 5&nbsp;V logic.  However, there is a 3V3 pin at the bottom, second pin from the left, that you could use:



However, unfortunately, the pin isn't protruding above the shield, so you'll have to make an ugly solder link to the top of the pin. Either that, or remove the row of pins and replace with a female header (carefully avoiding the SD card pins):





As an aside, the Iteaduino MEGA 2560 works at 3V3 but unfortunately the Mega shield that you have won't fit it. 



The Uno equivalent of your shield will fit it though: