When you add a Layout Managed Subsite instance, or convert an existing subsite, the wizard will be automatically run when you first visit the subsite. If you don't complete the wizard, the next time you visit you might see a blank page. To continue configuring the subsite in this case, visit the mounted subsite's admin directory, and choose the layout option. From that page, you can restart the wizard if you want (you can run any number of times), or visit the individual admin pages.