I'm attaching a screen shot here to show what I see when I click on the Navigation Bar button when Site Builder is turned on.This is in beta 26, which upgraded 4.1.11 PL3, and it's using the default vB style.
After watching the vB YouTube video of how to add nav buttons, I can tell this isn't how it's supposed to look. Although when I click in the large white area at the bottom of the left box, I do get the popup to add a new nav item, but something tells me I should probably be able to read the items in these boxes.