When you paste the objects into the "my_objects" folder, you just put them into the game so you can load em. You do not complete the whole process to actually get them into the map maker by doing this, lets just see it more as a "first step".
This is what the object maker is for, it lets you import meshes and particles and then save it into an object group, which on it's own hand work as the link to the map maker, since the map maker uses the object groups to place these objects into the map.
Simple mindmap
Objects into my_objects folder>Objects loaded in the object maker> Objects saved into an object group inside of the object maker>Load an object from the object maker in the map maker.
What you need to do is to open the my_objects folder and then read the names of the mesh files.
For example, if you downloaded meshes, and one of them is a tree, the creator maybe called the mesh. "LeafyTree". This file is a mesh file. If you open the object maker there's a mesh field that asks for the name of the mesh, and in the field you add "LeafyTree.mesh", and then choose add new.
After you have a visible mesh, you also need to save it into an object group, and make sure you make a new one, and do not save it into the standard ones like MyTress or MyRocks. Instead make your own like, VickoniObjects or something.