cant slide GUI slider

hi, i have the following script for my pause menu and in this script i wanted to create a slider that will be able to change my field of view, any time the fov changes the slider moves to the right place but i cant move it to change the value, it only displays it, can anyone help me? thanks.

 var mainMenuSceneName : String;
 var pauseMenuFont : Font;
 var pauseEnabled = false;
 var FOV = Camera.main.fieldOfView;
 
 function Start(){
     pauseEnabled = false;
     Time.timeScale = 1;
     AudioListener.volume = 1;
     Cursor.visible = false;
 }
 
 function Update(){
 
     if(Input.GetKeyDown("escape")){
         
         if(pauseEnabled == true){
             pauseEnabled = false;
             Time.timeScale = 1;
             AudioListener.volume = 1;
             Cursor.visible = false;            
         }
         
         else if(pauseEnabled == false){
             pauseEnabled = true;
             AudioListener.volume = 0;
             Time.timeScale = 0;
             Cursor.visible = true;
         }
     }
 }
 
 private var showGraphicsDropDown = false;
 private var showFovDropDown = false;
 
 function OnGUI(){
 
 GUI.skin.box.font = pauseMenuFont;
 GUI.skin.button.font = pauseMenuFont;
 
     if(pauseEnabled == true){
         
         GUI.Box(Rect(Screen.width /2 - 100,Screen.height /2 - 150,250,250), "Pause Menu");
         
         if(GUI.Button(Rect(Screen.width /2 - 100,Screen.height /2 - 100,250,50), "Main Menu (will not save)")){
             Application.LoadLevel(mainMenuSceneName);
         }
         
             if(GUI.Button(Rect(Screen.width /2 - 100,Screen.height /2 - 50,250,50), "Change Graphics Quality")){
             
             if(showGraphicsDropDown == false){
                 showFovDropDown = false;
                 showGraphicsDropDown = true;
             }
             else{
                 showGraphicsDropDown = false;
             }
         }
         
         if(showGraphicsDropDown == true){
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 - 50,250,50), "Fastest")){
                 QualitySettings.currentLevel = QualityLevel.Fastest;
             }
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 ,250,50), "Fast")){
                 QualitySettings.currentLevel = QualityLevel.Fast;
             }
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 + 50,250,50), "Simple")){
                 QualitySettings.currentLevel = QualityLevel.Simple;
             }
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 + 100,250,50), "Good")){
                 QualitySettings.currentLevel = QualityLevel.Good;
             }
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 + 150,250,50), "Beautiful")){
                 QualitySettings.currentLevel = QualityLevel.Beautiful;
             }
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 + 200,250,50), "Fantastic")){
                 QualitySettings.currentLevel = QualityLevel.Fantastic;
             }
             
             if(Input.GetKeyDown("escape")){
                 showGraphicsDropDown = false;
             }
         }
         
         if (GUI.Button (Rect (Screen.width /2 - 100,Screen.height /2 + 50,250,50), "Quit Game (will not save)")){
             Application.Quit();
         }
 
         if(GUI.Button(Rect(Screen.width /2 - 100,Screen.height /2 ,250,50), "Change FOV")){
             
             if(showFovDropDown == false){
                 showGraphicsDropDown = false;
                 showFovDropDown = true;
             }
             else{
                 showFovDropDown = false;
             }
         }
         
         if(showFovDropDown == true){
             FOV = GUI.HorizontalSlider(Rect(Screen.width /2 + 150,Screen.height /2 - 50,250,50), Camera.main.fieldOfView, 40, 90);
 
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 ,250,50), "40")){
                 Camera.main.fieldOfView = 40;
             }
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 + 50,250,50), "50")){
                 Camera.main.fieldOfView = 50;
             }
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 + 100,250,50), "60 (default)")){
                 Camera.main.fieldOfView = 59;
             }
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 + 150,250,50), "70")){
                 Camera.main.fieldOfView = 70;
             }
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 + 200,250,50), "80")){
                 Camera.main.fieldOfView = 80;
             }
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 + 250,250,50), "90")){
                 Camera.main.fieldOfView = 90;
             }
             
             if(Input.GetKeyDown("escape")){
                 showFovDropDown = false;
             }
         }
     }
 }