Hi Pascal,
in the Mixes panel select mix type model screen in the type pop options at the top of the mix options window and then at the bottom of the window at Model screen Options select the model you want to work with(by default this says no model but for example select the cube obj
That patches your your model into a mix. Patch a layer to the mix and choose some media so you can see how it is mapped (layer 1 for example)
Then in the libraries pane select the models tab and again choose the model you want to work with from the drop down tab.
Set your camera view
Tip
If you click On the groups cog you can show the xyz axes to better visualise
Map your texture to the model or model groups
In the display drop down in groups select layer 1 or whichever layer or mix you want to use as a texture
You probably want to flip winding in the texture controls unless you are "inside" your model.
You can then select texture mapping from texture controls.
Try this with the onboard models first, but you can import custom 3d obis into the model folder in the resources in the catalyst package contents
You can use a program like C4d, strata Lightwave or even sketch up to make uv mapped 3d objects
hope that helps screen grab attached..