In-Game GUI buttons don't work

Please, try this code and let me know if this work, because this works for me

 using UnityEngine;
 using System.Collections;
 
 public class InGameGUI : MonoBehaviour 
 {
 public string siteAdress;
 public bool mainMenu = false;
 public bool isQuit = false;
 public bool doWindow0 = true;
 public bool doWindow1 = true;
 private bool isPaused = false;
 Texture2D resumeTexture;
 
     void  Update()
     {
        if(Input.GetButtonDown("Pause") && !isPaused)
        {
          print("Paused");
          Time.timeScale = 0.0f;
          isPaused = true;
          GetComponent<MouseLook>().enabled = false;
          GameObject.Find("First Person Controller").GetComponent<MouseLook>().enabled = false;
        }
        else if(Input.GetButtonDown("Pause") && isPaused)
        {
          print("Unpuased");
          Time.timeScale = 1.0f;
          isPaused = false;
          GetComponent<MouseLook>().enabled = true;
          GameObject.Find("First Person Controller").GetComponent<MouseLook>().enabled = true;      
        }
     }
 
     void DoWindow0(int windowID) 
     {
        if(isPaused == true)
        {
          GUI.Button(new Rect(10, 550, 280, 20), "Quit");
        }
        if(isPaused == true)
        {
          GUI.Button(new Rect(10, 520, 280, 20), "Back To Main");
        }
     }
 
     void DoMyWindow(int windowID) 
     {
         if(GUI.Button(new Rect(10, 550, 280, 20), "Quit"))
        {
            
            Debug.Log("Quit Button Click");
            //Do not work on editor
            Application.Quit();
        }
 
        if(GUI.Button(new Rect(10, 520, 280, 20), "Back to Main"))
        {
         Debug.Log("Menu button click");
         Application.LoadLevel("Menu");
        }
     }
 
         void OnMouseDown()
         {
          if(isQuit)
             {
                  Application.Quit();
             }
 
             if(mainMenu)
             {
                 Application.LoadLevel("Main Menu");
             }
         }
     Rect wRect = new Rect(0,0,300,595);
     void  OnGUI()
     {
        if(isPaused == true)
        {
          
           wRect = GUI.Window(0, wRect, DoWindow, " Main Menu");
          
        }
     }
 }

i tried that but i keep getting errors