You go to the Level Editor. In the bottom, where all the items are for you to choose to build your map, there is a button called "Prefab". When you have subscribed to this and it should show up there. Then you pick it and place it. You don't see a blue hologram for prefabs. When you place it, it should be posted into the level editor. Then you can move it where you want.

When you select it, you can unpack it in the top left corner in settings. When you did so, you can change it as you want.

how do I use it I am stuck ._.