If you are having problems, try manually loading families like this: “Hi Russ. I believe the issue is that you first need to load the Site Designer families (from C:ProgramDataAutodeskSite Designer for Revit 2015Library) into the project before you try creating roads/walks/etc., and then convert your toposurface into a Base Toposurface (via the Set Base Toposurface button). After that, I was able to create roads, walks, retaining walls, etc. Hope that helps. Ross Kirby” via Revit 2015 R2 – Site Designer – Autodesk Community
or it might be visibility related: “got it working. I have mass unticked by default in visibility view graphics. all works after turning on mass. Russ Green” (from the same thread)
