Castor oil will work to give you a nasty case of diarrhea but it does not 'induce' labor - especially if your body isn't ready. It is the baby that determines when labor will start. When the baby is ready, you will go into labor.
This isn't meant to sound belittle in any way, but feel fortunate that you don't have to be overdue. Some women (myself included) go over 40 weeks. I know those last weeks of pregnancy are tough, and you can't sleep, you are swollen, etc. But I promise it will all be worth it in the end. You may not believe it now, but you will miss being pregnant in a few months. Weird how nature works, isn't it?
Good luck!