After you put them in the terrains folder open FH and go to map maker. Now just type in the names of the heightmap and mask under the "world" tab, so:
Whatever you named the height map when you saved it would be typed in where it says "Terrain Height Map" and whatever you named the terrain mask as would be typed in where it says "Terrain Alpha Map"
~Example~
Terrain Height Map:
Whatever you named your height map.png
Terrain Alpha Map:
Whatever you named your terrain mask.png