Alright, so assuming you know how to make objects with mesh's...
Once you get an object to show up in the open space in the Object Maker, you need to save it into a group with a name by making up a new group or clicking an existing one and hitting 'Save'.
Then, once you do that, and you have saved all objects you want, hit 'Export' and the game will export that file group making it use able in the Map Maker. You should be able to go to your 'Objects' tab in Map Maker and scroll till you see the group name and everything should be in there to use.