Here's a simple checklist of things to do to create models that will absolutely show up under any circumstances.

1. Use polys only, no Nurbs, Sub-ds, or anything else.
2. Only use quad faces, no n-gons or triangulated faces.
3. Make sure your object is oriented toward the Z camera
4. Make sure to follow richards instructions on how to name the models exactly.
5. Make sure to output only the selected model not the entire scene.
6. check you scale, if it is in mm, your model may be so small that it will look like a pinhole. If you have been working in very small units, just double check that you aren't making models that are too small.
7. try to use double sided geometry, i.e not planes. Instead of a plane use a very thin cube primitive, this way you are guaranteed not to have any reversed faces.
8. If you need to use planes, turn on your normals and make sure they face the Z axis. Catalyst seems to orient the model towards the Z axis.
If you're still having problem try outputting your model without any shading or bump/displacement.
Z axis on many modeling packages is not height it is depth as in from the cameras point of veiw. Only in architectural and 3dmax is Z up and down. You can change this preference in most packages so that Z isup.

I know that if you follow these steps your model should work. Experiment though in your spare time and try all types of geometry and faces. I'm not saying that they won't work, I just know what will.

Anyone with any in depth 3d questions, feel free to pm me and post here.

Christian