OnMouseDown on what? Camera Movement Help.

Hi,
I've added the standard MouseOrbit script to my MainCamera which allows the mouse to control an orbit around a central object. However, I wish for the camera to only move upon mouse down.

I understand the script will only work on a mesh or GUI Element, and so the difficulty here is that the Scene contains only a camera and my central object. There's no central mesh to apply the script to.

How do I work this so I can mouse down to scroll the camera?

Thanks,
Dan