In order to get meshes inside the map maker, you first need to load them in the object maker, then save them to a group. And a tip is not to save them to one of the standard groups, make a new group for your custom objects.
In order to get something like a waterfall in the map maker,
Open up object maker, go to the mesh tab.
In the mesh field, write the name of the mesh and end it with .mesh then press "add new" and if you wrote the correct name, it should load the object in the object maker.
Once that's done. You need to save it.
Go back to the object tab, and write a random name for it, and in the group field, make up a new group name it should be placed in, or give the same name as an already existing group for it to get placed in.
So for example.
The waterfall you want, is called "waterfall1.mesh" inside the fh directory. So you need to write in waterfall1.mesh into the mesh field, inside the mesh tab, then press "add new".
Then save it into a group. Once that's done, you can load that saved object in the map maker.
If you have a mesh that's not in the game from the start.
You just place the files for that mesh inside the folder here
C:\FeralHeart\media\objects\my_objects
And then do the same thing as before.