Hard to describe. In the middle I guess. Just look for buses marked P, R or S and they are there. All the stewards I've heard know where it is so just ask them if you are confused.
There are at least 3 buses and someone seems to have had a word as one of them is generally a double decker now too. The only thing I would say is that the buses are usually full by the time they reach the top of Woodingdean so you might have a long wait if you are catching from there.