First, select File->Import 3D Model, and then choose the 3D file format you wish to import.

In here you can see I want to import a 3DS file into C3DPS. You can import a DirectX 3D file (.X) in the same way.
After you select the 3D file format to import, the Open dialog will appear.

You can now choose the import type as C3D or D3D.
If you choose to import the object in C3D type, then it will only import the object mesh from the selected file and apply current material settings in the C3D project on the object. If you choose to import the object in D3D type, then the materials such as textures and mesh colors in the 3D file will also be imported.
You can click on the Preview button to see a preview in the Open dialog. The Preview feature can also be used to verify if the 3D model file can be loaded in C3DPS.
Clicks on the Open button then you will see the imported object model to be appeared in the C3D project window.

If the 3D model file is not compatible with C3DPS, C3DPS will give you an error to tell you that the file cannot be imported. In such case you can use DeepExploration which is been developed by Right Hemisphere to convert the 3D file to a C3DPS compatible format (.X or .3DS).
DeepExploration by Right Hemisphere:
http://www.righthemisphere.com/products/dexp/
And DeepExploration allows you to break up 3DS (or DirectX) models into separate parts and let you color them separately. Then you can use DE to re-package the models or save your separate pieces in 3DS / DirectX format and import into C3DPS.
As you can see in the project window that the object I imported here is a little dark. And the reason to cause this is because the ambient light in the project is been set to black. So I open the Attribute Panel and switch to the Light panel to adjust the ambient light.

Clicks on the Color button and choose the white color, the object will appear like this:

And it's a good looking fish.
You can download the fish model from http://www.planit3d.com/source/meshes_files/fish8.html
Hope this helps.
H.T.
