When I click on 'play' by the main menu, level 1 won't start... How to fix that?

public var welcomeLabel : GUIStyle;
public var scoreLabel : GUIStyle;
public var customSkin : GUISkin;
public var welcomeRocket : Rect;
public var playGameRect : Rect;
public var quitRect : Rect;

function OnGUI(){
  

    Screen.lockCursor = false;
    Time.timeScale=1;   
    
            var scale : Vector3 = new Vector3(Screen.width / 1280.0, Screen.height / 1024.0, 1.0);

            GUI.matrix = Matrix4x4.Scale(scale);                                                                      
                        
            GUI.Label(welcomeRocket, "ParkingGame",welcomeLabel);
                
            GUI.skin = customSkin;
             
                        
                 GUI.skin = customSkin;        //1
                if(GUI.Button(playGameRect, "Play")){
                 
                    Application.LoadLevel(1);
                }
            
                        
           
                        
            if(GUI.Button(quitRect,"Exit")){
                Application.Quit();
            }
                        
        }

Check if you have included the scene for your level one in build settings.

have you added your scenes in File -> BuildSettings?

Application.LoadLevel(1);

references the second entry in the list…