Well that was fun!
I only got about half way as it was more of a rope climbing excercise than tree climbing.
There was a rope over a pulley at the top of the tree. One end attaches to the harness and the other end passes through a sliding loop. The friction through this loop stops you slipping. Lower down there is another sliding loop which has a larger loop for your foot to go into.
A combination of pushing down with your foot and sliding the first loop up allows you to slowly climb up the rope two or three feet at a time.
I got half way, my son got a bit higher but we were both put to shame by an eight year old girl who managed to climb to the top twice whilst we were hanging around.
I'm going to have another go soon but I may need to get a bit fitter first!
Steve.