I'm sorry to say but I'm pretty sure that you can't make items in Object Maker.
You need to know how to 3D model if you want to make items. I would recommend using a program called Blender, it's free
You should take a look at this tutorial below. It's not up to date with I say you should get used to modeling before trying to make items.
https://www.feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=11025.0Blender 2.49 is the most compatible version with the Ogre exporter and importer. So here,
http://www.mediafire.com/download/jk7dnpj65khlj43/Blender_Foundation.zip I uploaded this from my computer. It already comes with the correct version of Python and includes a whole lot of exporters/importers. Just open it and move the folder to your
local disk drive (C:) or which ever one you use.
Now, 3D modeling is not easy. I would suggest watching a few video tutorials on modeling with
Blender 2.49 before attempting to make items. Once you get used to modeling with Blender then I can provide some more helpful links and programs you may need
If you are wanting a harness item though then I will have a go at making a harness item for you