Right now I'm at work without my mighty mouse, and I don't really remember which finger I use. I think I use the index finger.
Try using the mouse with slightly different grips and use the scroll ball with different finger. Eventually you'll fall into the most comfortable grip.
My might mouse feels like a natural extension of my arm now, but in the beginning it felt a bit strange, so I can relate to how you feel. Oh, and I turned off right-click, because I found then "natural extension" feeling to go away a bit with right-click turned on.