Hm. I'm not big on map making and, to be honest, have not made one myself.
But this might answer your question and solve your problem~
How To Make A Heightmap (http://www.feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=22787.0)
Hope you find the answer!
The blank screen is just the canvas. It basically like a piece of paper and you need to draw your map design on it.
Make sure you save the image as a .PNG and to also flatten the image. When you save the image it will come up with two options. Click the option to 'Flatten image'. If you don't flatten it then it is possible that your map maker will crash.