GUI box problem

So I'm looking at GUI Basics It says if I use this code:

 /* Example level loader */
 
 function OnGUI () {
     // Make a background box
     GUI.Box (Rect (10,10,100,90), "Loader Menu");
 
     // Make the first button. If it is pressed, Application.Loadlevel (1) will be executed
     if (GUI.Button (Rect (20,40,80,20), "Level 1")) {
         Application.LoadLevel (1);
     }
 
     // Make the second button.
     if (GUI.Button (Rect (20,70,80,20), "Level 2")) {
         Application.LoadLevel (2);
     }
 }

That it would do what it shows in the picture. So i copied and pasted, and the i tryed to attach the script to my player, I got an error message saying I can't add it because I haven't fixed all the complier errors. So the compiler errors are saying that 'Box' and 'Button' are not members of GUI. Is there just a problem with my Unity? or do I have to install this?