How do I correct the following error on the Lerpz Tutortial GUI

The GUI Script runs but does not display the buttons correctly and gives me this error -

MissingMethodException: Method not found: 'UnityEngine.GUI.button'. Boo.Lang.Runtime.DynamicDispatching.MethodDispatcherFactory.ProduceExtensionDispatcher () Boo.Lang.Runtime.DynamicDispatching.MethodDispatcherFactory.Create () Boo.Lang.Runtime.RuntimeServices.DoCreateMethodDispatcher (System.Object target, System.Type targetType, System.String name, System.Object[] args) Boo.Lang.Runtime.RuntimeServices.CreateMethodDispatcher (System.Object target, System.String name, System.Object[] args) Boo.Lang.Runtime.RuntimeServices+c_AnonStorey12.<>m_6 () Boo.Lang.Runtime.DynamicDispatching.DispatcherCache.Get (Boo.Lang.Runtime.DynamicDispatching.DispatcherKey key, Boo.Lang.Runtime.DynamicDispatching.DispatcherFactory factory) Boo.Lang.Runtime.RuntimeServices.GetDispatcher (System.Object target, System.String cacheKeyName, System.Type[] cacheKeyTypes, Boo.Lang.Runtime.DynamicDispatching.DispatcherFactory factory) Boo.Lang.Runtime.RuntimeServices.GetDispatcher (System.Object target, System.Object[] args, System.String cacheKeyName, Boo.Lang.Runtime.DynamicDispatching.DispatcherFactory factory) Boo.Lang.Runtime.RuntimeServices.Invoke (System.Object target, System.String name, System.Object[] args) StartMenuGUI.OnGUI () (at Assets/StartMenuGUI.js:22)

Here is my script (The error is on line 22)-

 //Make the script also execute in edit mode
 @script ExecuteInEditMode()
 
  var gSkin : GUISkin;
  var backdrop : Texture2D;    //our backdrop image goes here.
  private var isLoading = false; //if true, we'll display the "Loading..." message.
  
  function OnGUI()
  {
      if (gSkin)
          GUI.skin = gSkin;
      else
          Debug.Log("StartMenuGUI : GUI Skin object missing!");
      
      var backgroundStyle : GUIStyle = new GUIStyle();
      backgroundStyle.normal.background = backdrop;
      GUI.Label ( Rect((Screen.width - (Screen.height * 2)) * 0.75, 0, Screen.height * 2, Screen.height), "", backgroundStyle);
      
      GUI.Label ( Rect((Screen.width/2)-197,50,400,100), "Lerpz Escapes", "mainMenuTitle");
      
      if(GUI.button( Rect((Screen.width/2)-70, Screen.height -160,140,70),"Play"))
          {
              isLoading = true;
              Application.LoadLevel("TheGame");
          }
  
  var isWebPlayer = (Application.platform == RuntimePlatform.OSXWebPlayer || Application.platform == RuntimePlatform.WindowsWebPlayer);
  if(!isWebPlayer)
      {
          if(GUI.Button( Rect((Screen.Width/2)-70, Screen.height -80,140,70), "Quit")) Application.Quit();
      }
  
  if(isLoading)
      {
      GUI.Label ( Rect((Screen.width/2)-110, (Screen.height/2) -60,400,70), "Loading...", "mainMenuTitle");
      }     
 }

If anyone can tell me what the error means and what I should be looking for to fix it that would be great.

Thank you!