GUISkin Problem

i write menu script easily, but i cant to put any GUISkin, there is my code.

 /* GUI.Button example */
 
 var GUISkin:GUISkin;
 // JavaScript
 function OnGUI () {
 GUI.Box (Rect (140,90,180,230), "MENU");
     if (GUI.Button (Rect (170, 120, 120, 30), "Play Alone")) {
         // chatvirtviskodi
     }
         if (GUI.Button (Rect (170, 160, 120, 30), "1 VS Computer")) {
         // chatvirtviskodi
     }
             if (GUI.Button (Rect (170, 200, 120, 30), "1 VS 1")) {
         // chatvirtviskodi
     }
                 if (GUI.Button (Rect (170, 240, 120, 30), "Multyplayer")) {
         // chatvirtviskodi
     }
                     if (GUI.Button (Rect (170, 280, 120, 30), "Exit")) {
         // chatvirtviskodi
     }
     
 }