You'd be better off asking your map-maker how to do it, but here're some ideas.
Firstly, in case you haven't restarted FeralHeart after downloading, you need to restart FeralHeart in order for it to work.
If you did or this doesn't work, sometimes I find it helps to copy the map file (.fhm) and put one in Exports and one in Terrains (found under FeralHeart>media>terrains).
If it's still not working, then it's probably not worth continuing trying, unless you really need the map for a roleplay or something, in which case you should ask others if they've had the same problem with the map and if they have, ask the map-maker to re-export it for you.
