I have a 2011 deployment and we are testing out 2013 before we migrate.
I have quite a few custom entities that are found in my settings > extensions in 2011.
When I create new entities in 2013 and even though the checkbox for the entity to display in the settings area. (found under customize the system entity general tab)
The new entity cannot be found. In fact extensions is not even an option in the new dropdown navigation ui when in the settings category.
All of my new entities are published of course.