int ledPin = D2;

void setup() {
      // We want to tell the Argon that we'll use
      // D2 as an output pin.
    pinMode(ledPin, OUTPUT);
}

void loop() {
    // First... On
    digitalWrite( ledPin, HIGH ); //turn LED on
    delay(3000); //wait for 3 seconds
    
   // Now...Off
    digitalWrite( ledPin, LOW ); //turn LED off
    delay(3000); //wait for 3 seconds
    // rinse + repeat
}