Currently, the easiest CAD format to import into the DIRSIG environment is the Alias/Wavefront OBJ format. At RIT, we currently use Rhinoceros (http://www.rhino3d.com) and Blender3D (http://www.blender3d.com) and export the model to the Alias/Wavefront OBJ format.
If your CAD environment does not support export to this format, you can consider translating to this format using third party software like "Deep Exploration" available from Right Hemisphere (http://www.righthemisphere.com).