What I do wrong ? (OnMouseDown)

What I do wrong ?

using UnityEngine;
using System.Collections;

public class OMD : MonoBehaviour 
{
	public GameObject Object ;

	void OnMouseDown(Object.gameObject)
	{
		Debug.Log("Done");
	}
}

void OnMouseDown(Object.gameObject)
to

void OnMouseDown ()

And also make sure you can actually click on the object that contains this script (e.g. GUI, collider component).

You can check out THIS video tutorial for visual guidance.