Here is my code for doing something equivalent. You seem to have the pinMode line missing in setup()

// Initialize motion message
MyMessage msg(CHILD_ID, V_TRIPPED);

void setup() { 
  pinMode(MOTION_SENSOR_PIN, INPUT_PULLUP);      // sets the motion sensor digital pin as input
}

void presentation() {
  // Present locally attached sensors here    
  // Send the sketch version information to the gateway and Controller
  sendSketchInfo("Motion Sensor", "1.1");

  // Register all sensors to gw (they will be created as child devices)
  present(CHILD_ID, S_MOTION);
}

boolean pinstate = false;

void loop() {
  boolean newstate = (digitalRead(MOTION_SENSOR_PIN) == HIGH);
  if (newstate != pinstate) {
    if (newstate == true) {
      send(msg.set("1"));
      Serial.println("Motion Started");
      pinstate = newstate;
    } else {
      send(msg.set("0"));
      Serial.println("Motion Stopped");
      pinstate = newstate;
    }
  }
}