Look for the big black C in a yellow box on the wall of an office. Wander in there and you should find the booking in/out book there. If you are lucky there may even be a pen there! We all book in/out on the little grass strip I operate from even if it's just a local flight. I'm not sure if it is a legal requirement or not. Have a search around in Pprune as I'm fairly sure this has been asked before.