To make something spawn inside the painted cells, you need to create a BuidlingLogic object and give it to the tool.
This object will store the rules that the tool will follow to spawn actors inside the cells.
This makes the tool really modular and powerfull : It's a tool integrated to the editor but you can write your own logic in it to modify its usage. And you can do it entierly in Blueprints !
That way, you can choose to setup the spawning logic to create a platformer map or a house, a dungeon, a cave,...
The object which is responsible for spawning actors in cells is the BuidlingLogic object.
To create a new building logic, create a new blueprint which inherit from the DungeonBuildingLogic class :
You need to override the SpawnContentInCell event. This is the event which is called to spawn cell content.
Here is the content of the SpawnBaseLayer function. It will simply spawn floor meshes, walls or doors and some columns in the corners.
Get the example content to play with it.