Answer by diabloroxx

Check this for Rotating an object: http://unity3d.com/support/documentation/ScriptReference/Transform.Rotate.html

Check this for getting Input and identifying them:

Try this:

void Update()
{
 if ( Input.GetMouseButton( 1 ) )
        {
          Camera.mainCamera.transform.RotateAround(Vector3.zero, Vector3.up, 20 * Time.deltaTime);
        }
}