I wanted this to be a 'drop in' replacement for my mousehandler, so here's the script I put on my prefab:

if (Input.GetMouseButtonDown(0)) // check for left-mouse
{
    var ray = Camera.main.ScreenPointToRay (Input.mousePosition);
    var hit : RaycastHit;
    if (collider && collider.Raycast (ray, hit, 100.0))
    {
        OnMouseDown();
    }
}

var ray = Camera.main.ScreenPointToRay (Input.mousePosition); var hit : RaycastHit;

if (Physics.Raycast (ray, hit, 200)) 
{
    var objecthit:Transform = hit.transform as Transform;

    // What are you hitting?
    if (hit.collider.tag == "3DButton") doStuff()
     }

You can also use OnMouseDown(), which is easier, but the object has to have a collider, I believe.