You may have noticed the words "Load Level" down at the bottom left hand corner of the demo. You can click those words and load in a *.png file as a level. You can use the key to the right to draw your map and indicate the top-left corner of where you want any game object to spawn. Currently this feature is limited in that you can't draw backgrounds or hidden (no_collide) tiles, and I expect a number of bugs to be reported. Enjoy, and leave comments with links to your levels!
Here are some sample levels: