Problems with changing scene via the use of a button

Hey, So I'm totally stumped over my problem. I have written the following code below:

 using UnityEngine;
 using System.Collections;
 
 public class PlayGameButton : MonoBehaviour 
 {
     public void NextLevelButton(string levelName)
     {
         Application.LoadLevel(levelName);
     }
 }

I have created an empty game object and attached my script to it. I have clicked the "+" icon on the button script component and dragged and dropped my empty game object containing my script onto the object box. I have selected the function ButtonNextLevel.NextLevel(string). I then fill in the text scene "MainMenu" which is the name of the scene I want to transition too. And then... nothing happens...

Now I have added it to the build settings and YES it is spelt correctly! :D

An the other thing is that all my buttons work on the main menu screen to transition to controls/instructions/play game etc, but from any other screen... NOPE it does not want to work.

P.S. I do also get this error after building and running the project - http://i.imgur.com/u1tgg7i.png