Make sure your height map is the following and entered into mapmaker the following way;
513x513 pixels
One layer
Greyscale
No alpha channel
PNG. file
Save the file as (MapNamehere)Terrain.png into the folder with the name of your map.
This means you need to go to Feral Heart Map Maker and save the map name first. For example, if you wanted it to be FarrahIsland, put that in the top dialog box, and then Farrah Island in the second, and then save. It should come up on the list with watermill.
The height map you made will need to be save in FH>Media>Terrains>FarrahIsland.