If I didn't want, for example, this roof to show, what I would do, is I would select this roof. I would right click, and I would hide it in view. We won't do it now, but what would happen is, Navisworks would not export that item that you don't want shown. Hit escape a couple of times to clear the selection. Now, what you need to do if you don't have the add-ins is, you need to install Revit first, and nava-script second. This will give you the available add-ins that you need.