They are not assets located in Surveyor. When you are in Surveyor, you already have opened (or started) a route and these M120 things are routes.
To download them, (re)start the game and pick “Manage Content” at the start. Next, make a filter in which you search for “Name = M120” and “Author = oknotsen”... since I happen to have made those 4 demo routes that come with the game and a few more modules.