You can relax. I went through the process recently and it's nothing to be scared or nervous about.
Idea generation was really asking us to think outside the box. So take your time, don't be phased by time pressure and come up with something origional !
Case study's are like the on-line example. Make use of figures and there is always too much info to read through, skim if you can.
In the group session make sure you have a role in the group. Best to be the time keeper, get in there first with that. Having said that I was time keeper and had my watch to a different time to the one on the wall, but I still got through !!
Remeber to be considerate and supportive of your group memebers. And finally good luck !