I know this thread is quite old but it really helped me to prepare for my AC with KPMG yesterday so just thought I would share my experience as it's more up to date.
All the advice on this thread is still really relevant as it is exactly the same AC but I did read on the target jobs website that they recommend in the self review at the end of each exercise that you don't talk too much about the positives as the assessors are going to see what you've done well anyway. I don't know if this has changed but now there are designated sections of what you did well, what you didn't do well and what you would do differently next time. This is a timed exercise and 10 minutes goes by quick so get your points down and then go back to flesh them out if needed. Be honest here as one of the key competencies is 'strives for continuous improvement' so they want to know you're aware of your strengths AND weaknesses.
The laptops are quite a lot smaller than a standard laptop (mine is a dell inspiron which is quite a lot larger) so you will need to get used to where the keys are and constantly check for typos as there is no spell checker.
I would also recommend you get a friend to practice role plays with - it doesn't matter what scenarios you do just get used to these situations. This was my first AC and I was thrown by nerves especially in the first role play. KPMG use professional actors for the 'client' and 'manager' role but when I get nervous I want to giggle and everything about the situation is very alien. However how well the actress played the role of the client threw me again and put me on the back foot and I think in places I lost control of the situation. I was much more prepared for the second meeting and felt I did a lot better in that.
Other than that just try to relax and enjoy yourself. I was extremely nervous as it was my first AC and I didn't manage to get a lot of sleep. When I walked in I was ten minutes early and still the second to last to arrive. I was the only girl amongst four other guys who all wore suits and most were older than me. Although initially I felt very out of my depth none of them were in the same stream as me and the facilitator said at the beginning on a good day we could all be successful and on a bad day we could all be rejected so at no point do you feel you're being pegged against someone else. The facilitators were both really friendly and were open to any questions. Even the assessors in the role play were friendly and made you feel a little less nervous.
I'm still yet to hear back and I'm worried I stuffed up the first meeting. Will be absolutely gutted if I failed but nonetheless it was a really great experience! If anyone wants any more info feel free to PM me or reply on the thread