The content of the Inheritance Strategy dropdown box you observed is expected for the base class in the structure. It will by design show only the Default and Horizontal options. The Flat option will appear in this dropdown box only for the child classes. In this article
you can find a how-to guide that will help you to create such a structure in your model.
I hope this helps. If you need further information, do not hesitate to get back to us.
OpenAccess ORM is now Telerik Data Access
. For more information on the new names, please, check out the Telerik Product Map