I don't really have map making experience but I'll try my best to help and explain. If you've already downloaded the custom objects I believe you put them into your My Objects folder (C:\>Feralheart>Media>Objects>MyObjects ). Then you open up Feral Heart and go to the object maker. Type whatever the file names are that way you can load them into the object maker :) Just be sure to type the name in correctly.
Check out this tutorial from another helpful user it might help ^_^:
http://feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=12012.0 (http://feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=12012.0)
Specifically this part though:
Installing onto Feralheart
Open up FH and go onto Object Maker.
This is where you type the mesh file in. So the mesh i downloaded was "trebol20.mesh" and "trebol1.mesh" so i'd type in one of them in the "Mesh" box, then click the "Add New" button and it will appear.
This is where the mesh's material is you can also change it too another material if your not happy with it.
What ever you download you will walk right through it. Clicking this will prevent that so you can stand on it and and not walk through it
(http://i55BannedImageSite/30lgrw1.jpg)
Now you would want to save your mesh so you can use it in maps.
This is where you would put the mesh's name i called it clovers
Here is where you choose the group to put your mesh in, I made a a downloads folder to put my mesh's in.
Here is your object folders where your objects are stored. There underlined in blue is the my "Downloads" folder where ill find my mesh.
Click the "Save" button, then you can find your object folders with its contents in Map Maker.
(http://i52BannedImageSite/2v2e3pk.jpg)
I hope this tutorial helps, and if you need any more information about Object Maker ill be happy to help ;)
I hope that helps :)