Instructions
Creating a Room
Click the "Prefabs" tab, at the bottom of the "Library" panel and select "[add new prefab]." You must add one room to your level before you can place an Entity.
Click on a desired room image and select "OK." Left-click a point on the level grid to place the room. You will notice that the transparent room blueprint trails your mouse cursor, as you move it.
Press the right mouse button to clear the "Prefab" from your cursor. You are now ready to create an Entity.
Adding an Entity
Click the "Entities" tab at the bottom of the "Library" panel found on the left side of the interface. Select the "[add new entity]" item that now appears on the "Library" panel.
Choose an Entity from the list of directories that appear. Select anything from furniture to characters by clicking an entry with the left mouse button. A new menu will open.
Left-click the image representing the Entity you wish to add to your level. Click "OK" to confirm your selection.
Rotate an Entity in 45 degree increments by pressing the "R" key. You may also hold the "Shift" key and press "R" for a more gradual tweak to the object's rotation. Press the "B" key to cycle between free Entity placement, having the object snap to the level grid, or having it align with geometry units.
Click the desired target area of your level with the left mouse button to create the Entity. The object will now appear in your level, behaving as you would expect (e.g. monsters attack the player, lights illuminate a room, etc.)