OnMouseDown is only called when the user has pressed the mouse button while over the GUIElement or Collider. I think you want GetMouseButtonDown. You can also access the main camera via Camera.main.

function Update ()
{
   if (Input.GetMouseButtonDown(0))
   {
      Camera.main.transform.Translate(1,1,1);
   }
}

Your code is correct. Just make sure that you add a collider to the Plane (which you want to assign as a button). You can also add a GUI button to translate or move your camera. That way your button will always be in view. I assume you will not want to move the Plane along with your camera.

Add collider by going to Physics and then collider.