On the "Particles Treeview" section, expand the Editor Properties, then Backdrop, then Right Click "3d Layers(Scene)", select New Backdrop, then CNEdEditorBackdrop_Model3D.
In the section that just became visable, under General, select your model there.
If you accidentally closed any of these windows, you can reanble them from the "Layout > View" menu on the upper toolbar. You may have to untick and retick them (I think this is a bug).
I'm really new to popcorn so please excuse if this is misinformation:(