Firstly, you need to make a 60x60px image for the inventory. I usually open the model in ModelView, get a good angle, and pick a contrasting background color. Turn off SSAO, Ambient Light, Bloom, and the floor as well. It should be a simple shape floating in a space. Hit "Prt Sc" on your keyboard, open an image editing software, and paste it in. Crop it so only the "PO" is in the space. Add an alpha channel, resize to 60x60 Pixels, and save as a targa (.tga). Save it in the same folder as the PO to keep organized. Then:
1) Open the ModelEditor and import the "P.O."
2) Edit > User Defined Variables
3) Type:: Item ...... SubType:
uzzle
4) Scroll to the bottom, give it a name that will be given in the extra_english.lang
5) Under it, locate the image you made.
6) Add a body around the mesh, attach it and tweak the body's variables as necessary and save.