So assuming I'm understanding your issue correctly, putting your terrain name into the mapmaker crashes your game?
If that's the case, the .png is important and should definitely be there when typing in your terrain name. The issue may lie with your heightmap.
Make sure it's:
-Grayscale mode
-513x513
-One layer
-No alpha channel'
-No spaces in the file name
If you're not using GIMP to make your heightmap, then it's most likely not in grayscale mode and will cause your game to crash.