Some users cannot select a floor from the Floor Selector. They could expand the buildings to see the floors, but not double-click to activate a floor.
This occurred in earlier versions of OpenBuildings Designer, usually when a high resolution display was in use. Slowing down the double-click speed in the windows mouse settings can improve the issue.
Another possible workaround is to delete the user preference files and then select a floor.
This problem has not been reported on current application versions.