Here's a repost from part of the FAQ at acadiaforum.net -
Where did GM put the battery?
You will need a 7/64 stardrive. Slide the front passenger seat all the way foreward. Remove the mat, and this is what you see
The plastic covering to the left can be snapped off, but I was afraid to try. I was able to slide the cover off. I think it will be a minor pain to drop the battery in, but certainly possible. But that plastic cover to the left will definately have to come off. Notice all the cables, fuses, etc.