That is probably because of the layer you put it on. Immediately to the left of the place/fill tools are the layer buttons. The first is for the bottom layer, the second for the middle, the third for the top, and the fourth for the 'Event' layer. Generally, you should put the ground on the bottom layer and everything else on either the middle or top layers. This will make the objecs seem that they are on top of the ground, and there won't be that black area. Also, you can't place objects on the 'Event' layer. That layer is for after the map is finished and you want the specific objects to do something.