How to add "Press ESC to EXIT"? and exit the game?

Hello,
To my spaceshooter tutorial game I want to add an Exit/Closing the game function.
How do I do that?

I’m having trouble.

This is the “Done_GameController” script in which i’ve added
if (Input.GetKeyDown(KeyCode.Escape))
{
Application.Quit();
}

Pressing ESC do nothing, I don’t undersatand why?

using UnityEngine;
using System.Collections;

public class Done_GameController : MonoBehaviour
{
public GameObject hazards;
public Vector3 spawnValues;
public int hazardCount;
public float spawnWait;
public float startWait;
public float waveWait;

public GUIText scoreText;
public GUIText restartText;
public GUIText gameOverText;

private bool gameOver;
private bool restart;
private int score;

void Start ()
{
	gameOver = false;
	restart = false;
	restartText.text = "";
	gameOverText.text = "";
	score = 0;
	UpdateScore ();
	StartCoroutine (SpawnWaves ());
}

void Update ()
{
	if (restart)
	{
		if (Input.GetKeyDown (KeyCode.R)) {
			Application.LoadLevel (Application.loadedLevel);
		}
		if (Input.GetKeyDown(KeyCode.Escape))
			{
				Application.Quit();
			}

	}
}

IEnumerator SpawnWaves ()
{
	yield return new WaitForSeconds (startWait);
	while (true)
	{
		for (int i = 0; i < hazardCount; i++)
		{
			GameObject hazard = hazards [Random.Range (0, hazards.Length)];
			Vector3 spawnPosition = new Vector3 (Random.Range (-spawnValues.x, spawnValues.x), spawnValues.y, spawnValues.z);
			Quaternion spawnRotation = Quaternion.identity;
			Instantiate (hazard, spawnPosition, spawnRotation);
			yield return new WaitForSeconds (spawnWait);
		}
		yield return new WaitForSeconds (waveWait);
		
		if (gameOver)
		{
			restartText.text = "Press 'R' for Restart";
			restart = true;
			break;
		}
	}
}

public void AddScore (int newScoreValue)
{
	score += newScoreValue;
	UpdateScore ();
}

void UpdateScore ()
{
	scoreText.text = "Score: " + score;
}

public void GameOver ()
{
	gameOverText.text = "Game Over!";
	gameOver = true;
}

}