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) = gSkin;
 		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;
 var isWebPlayer = (Application.platform == RuntimePlatform.OSXWebPlayer || Application.platform == RuntimePlatform.WindowsWebPlayer);
 		if(GUI.Button( Rect((Screen.Width/2)-70, Screen.height -80,140,70), "Quit")) Application.Quit();
 	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!

Looks like capitalization to me :). I don’t know anything about GUI.buttons, but I do know that GUI.Buttons work.