Once you have put all their files in my_objects, you need to launch FeralHeart, and then go to Object Maker. Once there, tab over to the mesh tab. There, you must type in your meshes name, and .mesh after it, like so,
Then, once it's up, make sure it's using the right materials.
Then tab over to Object, type the name of the Object, ie, RockCave,
then for Group, you MUST create your own. Meaning not MyTrees, MyRocks, etc, etc.
Do this by typing in a group that does not currently exist. Pressing save will create it for you, and then it will show up in Map Maker.