How to make a menu for picking a level to play

I am trying to make a menu that you can pick what level to play with. I have used the game jam standard menu for my main menu but can't get the buttons on the level select screen to work. using UnityEngine; using System.Collections;

public class Level_select : MonoBehaviour {

     // Use this for initialization
 void OnMouseDown () {
     Application.LoadLevel("Breaker");
 }

}

This is the script i am trying to use, It has no compiler problems yet it will not load the level.