This quote should help. Make sure you're following everything there:
If you keep trying to type the heightmap into the mapmaker, it may crash if certain things weren't done correctly such as:
~513x513
~grayscale
~no alpha channel
~only one layer
~named "mapnameTerrain"
Also, try changing your rendering system and go into FeralHeart >>> media > particles and delete any ending in _0, _1, and so on. And remember, sometimes using complex brushes for a height map will crash the game because the game cannot render it's complexity.
Also, to publish maps, click export with your map completed and get all of the files it spits out from Feralheart > exports. :3