Once you're on the moon, open your popit with SQUARE and select the icon that looks like a track with a wrench on it. Select whether you want to build a track or arena. If you're building an arena, then all you have to do is select a shape, save, and the trophy unlocks. If you build a track you have to pave the track out with the tracklayer. After you have rolled from the start point around back to the start point, save and the trophy will unlock.