Are you wondering how to use the Level Boundaries? scroll a bit into the void so you can see it (with the respective arrow key), then click the respective button "Top", "Left", "Right" or "Button", and click again into the void.
I'm trying to figure this out now for a couple of days. I was testing the latest version which rocked by the way. The Mario Challenge had such chars like for example Link in them. I wanted to replace them by for example Toadette and other direct SM chars. But other than sprites and some bad swaps, I didn't succeed. How to go about this?