Dave, its possible that the instructions don't include a rabbet. I believe there are models that don't include it in the directions but I am not sure with your particular model. Maybe IM people who have completed or are working on it.
I do know that some models I have seen here and I am not an expert but they create the rabbet using a strip of wood that is thinner than the bulkhead former that sits between the keel and bulkhead former. The Syren is a good example of this. You can download the directions from Model Expo for free, ch 1. show this. Other models have you carve the rabbet.
Sorry I can't be more help, but without the actual directions in front of me its just a guess. I believe some people have created rabbets even when the instructions don't mention it because it allows for a tighter fit of the planks.
Hopefully someone who is more of an expert can give you better advice.