The Adsense option simply inserts whatever code you insert there below the first three posts of your blog. It's actually a placeholder, really. You don't have to use Adsense, you could use it to insert a picture of your pet unicorn, if you wanted. ;) For a long time, I used it for both Adsense and another advertising platform at the same time.
On a related note, what's the difference in action hooks between graphene_before_slider and graphene_before_slider_root?
These simply refer to different locations with regard to the slider. Unfortunately, we haven't developed a "map" of the action hook widget areas (which we probably should in the documentation), so I have found the best way to figure things like this out is to activate both of them, add a text widget to each one, and in the text box of the text widget, type the name of the action hook widget area. Once you save them and view your page, it's easy to figure out where each one goes.
Hope this helps. Action hook widget areas are a lot of fun, once you find the right one.