A BookCrossing Zone is any place where someone has released a book. So if you release a book somewhere, that makes it into a BookCrossing Zone. That's all there is to it. Go ahead and release your book in a spot that's not been used before, enter it on the website, and hey presto! Your town now has a BookCrossing Zone.
... Reading your question again, I'm scratching my head. Registering a book does not require a BookCrossing zone. Only releasing a book does that. Which one are you trying to do? Please give us some more details, we'll be happy to help.
If I want to register a book, it wants me to select a BookCrossing zone. What if my town does not have one?
You can add a new zone if there isn't an existing one that's close enough to your release location. Do check first, though; the release-note process will let you select your town, and will display all the zones previously used in that town, whether there have been any recent releases there or not. [The "go hunting" pages only show zones where books have been released within the last 30 days, so there may be many more zones in your town than currently display on that page.]