OnMouseDown not firing

I've got this code, but when i press the pause button it will not execute it, if i check the Paused bool in the inspector it will execute it immediately, but how do i fix it to make it work?

 #pragma strict
 var paused = false;
 var isButtonVisible : boolean = true;
 
 function OnMouseDown(){
     this.paused = !this.paused;
     Time.timeScale = 0;
     isButtonVisible = true;
 }
 
 function OnGUI(){
     if ( isButtonVisible ) {
         if(this.paused){
             if (GUI.Button(Rect(Screen.width/2-100,Screen.height/2+3,200,50),"Restart")){
                 Application.LoadLevel(Application.loadedLevel);
                 Time.timeScale = 1;
                 isButtonVisible = false;
                 
             }
 
             if (GUI.Button(Rect(Screen.width/2-100,Screen.height/2-50,200,50),"Resume")){
                 Time.timeScale = 1;
                 isButtonVisible = false;
             }
 
             // Insert the rest of the pause menu logic
             if (GUI.Button(Rect(Screen.width/2-100,Screen.height/2+56,200,50),"Main Menu")){
                 Application.LoadLevel ("MainMenu");
                 isButtonVisible = false;
                 Time.timeScale = 1;
             }
         }
     }
 }