Look hun, for all the objects you've downloaded, make sure that you have made a separate section for the items. When you go into map maker, don't put them in the same section as default, my plants or the others. Make a separate section for the new objects. For a visual look at the image below:
(http://i43BannedImageSite/icsj7m.jpg)
Alright, once you've downloaded your files and put them into my_objects, you open up Object Maker in Feralheart. Using the name of the .mesh file, you type it into where 1 is. So, if the mesh file was called "treehouse.mesh", you type that exactly in, including the '.mesh'.
In 2, the mesh might fill it in automatically, but if it doesnt you will also need to put the exact name of the material in here to colour it. So, if the material is called "treehouseMat", thats what you put.
After that, name it, then make a group for it according to the map you're making. If you were making a "Herp Derp " map, then the object group could be called "HerpDerpObjects", etc. You can put many objects into the same group.