How to make a GUI button appear OnMouseDown

I am working on some code, that when an object appears a GUI box appears with some text.
However the issue is that, as soon as I run my game it appears and is not being triggered by my if statement. I only want it to appear, when the particular object gets clicked.

private var InfoBox = false;
var boxwidth: int = 300;
var boxheight: int = 300;
var boxfromleft: int = 500;
var boxfromtop: int = 500;

function OnMouseDown () {
	if (Input.GetMouseButtonDown) 
		InfoBox = true;
function OnMouseExit () {
	InfoBox = false;

function OnGUI () {
	if (InfoBox) {  	
	    GUI.BeginGroup (Rect(boxfromleft,boxfromtop,boxwidth,boxheight)); 
	    GUI.Box(Rect(0,0,200,200),"A Classic Neo-Victorian Turret");
	    GUI.Button(Rect(10,40,150,40),"Browse All Turrets");
	    GUI.EndGroup ();

Thanks for the help in advance

This line is not needed: if (Input.GetMouseButtonDown)