Hi @Marena,
By default, the root category is not displayed on the frontend, so it's not possible to use 'the root category page primary to display brand page' as you mentioned.
However, please try this workaround. You create an anchor category (for instance 'holder-category') which is the sub-category of your root category. Then you move all your sub-categories into this newly created 'holder-category'. Then you can display brand page with layered navigation.
Hope that helps.
Regards,
Ubertheme team