Also not sure if you know this, but every item you import using S3d Editor must ALWAYS ALWAYS be triangulated. If a model has four sided polygons it will not import. As a beginner I spent months wondering why my models wouldn't import.
In Wings3d you can take an existing model and right click "tesselate -> triangulate"; that way a four-sided square is converted to two three sided triangles and so on.
Another thing - with S3d Editor every model must have UV coordinates to be imported. One trick I use is to create several basic cubes, cylinders, & spheres, texture them, and then use them as the basis for every part of the model in wings3d. When the model is finished I can always touch up the textures so they are more fitting for a complex ship model.