It is not just you that is confused my the plan, I think every one of us has been bamboozled by it at some point. Which is why I started reading further. I great site is http://www.mendosa.com/gidigest.htm.
It clarifies a lot about the GI index system and GI load. From there - I got the book, New Glucose Revolution, which is similar to Montignac, and I find easier to digest... no pun intended!
As far as the yogurt goes. As long as the Yoplait you have found has no other sugars, it *should* be ok. Splenda is fine by Montiganc. Personally, I would still use plain yogurt, and sweeten it by:-
1. Adding fructose myself (easier to measure fructose than Stevia products)
2. Making my own fruit puree, or adding fruit to it.
I'd do this mainly as I try to avoid any artifical sweetners of any kind.
Good luck with Montignac, when you have interpretted the basics it is a great plan that I'd recommend to anyone. Feel fre to post any queries here - there seems to be a hard core of 5 of us (including you) that will be more than willing to help!