A resistor that ties the input to ground is a pull-down
, not a pull-up.
If you are talking about the Arduino ProtoShield (http://arduino.cc/en/Main/ArduinoProtoShield
) that button is connected to the Arduino's RESET. So you can't really use it for general purpose I/O.
You need to put a button somewhere else on the proto area, so you can connect it to Pin 2. It's up to you if you want to use an external Pull-Up (or pull-down). Personally, I would just enable the internal pull-up resistor and avoid the extra component. The only thing to remember is that when using a pull-up that HIGH = unpressed and LOW = pressed.