To make the terrain visible in the map system, follow these steps:
Assign the layer Terrain to the terrain gameObject.
Inside the Map System gameObject, there is a camera, to render the tile meshes created with the map creator. In that camera, add the layer Terrain in the culling mask.
Make sure to create at least an empty floor in the Map System component in the Character gameObject, it doesn't need to have any map part.
Then, the terrain will be visible in editor mode in the mini window and ingame.