Press 1 to add walls, press 2 to make spaces, and space to delete the latest blocks you placed. You can also press 3 to add multiple walls quickly, or 4 for multiple spaces. (m changes the volume) I know that the delete doesn't currently work. I'm currently working to find a fix. To save your dungeon, right click on DungeonGen list, and press export. To bring it back into scratch, right click on the DungeonGen, press import, and click on the file you got from exporting before.
Code by me. Finished the preview/cursor Music by Zealot13 and Jr_Monkeyman