GUI not showing in game

Hi, when i create a new GUI script and click play nothing shows up on the screen but dark blue, also I use a Mac OSX and my script is / 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);
 }

}