This looks really awesome! I'm thinking about buying it.
For those wondering how it works, here's a video of it from their blog (you should upload this to the asset listing BlueBurn)
https://www.youtube....h?v=FfugOlQKcFc (don't know/can't remember how to embed a YouTube video)
So it looks like there's a folder with rooms in it that is the editor. You move that folder to the top of the room resources list and run your game, which then runs the editor (so it's coded in GML/extensions). You make your 3D level then you close the runner, move the room that you edited with the 3D editor to the top and then run your game again. That's what I could gather from that video. It seems a little tedious, but at least it can be sold and downloaded in a Marketplace asset format.