Goto the track you want to create an Autocross track on, press shift + u. If you cant see any GUI, press space. Press shift + u to leave the editor.
Don't forget to turn on "Place Objects" in the little menu that pops up when you press shift+U. That will bring up another menu on the top right with all the objects you can place. the "o" key will place an object, "," and "." will rotate it.