Well, first off, I'm no expert. I haven't successfully imported any models into FreeSpace yet, but I am working towards it.
There are several ways of getting a model into FreeSpace. You can download TrueSpace for free.
http://www.soft82.com/download/Windows/TrueSpaceAnd import TrueSpaces models via the COB file format with the tools here.
Or if you can get your hands on 3DSMAX 5, 6 or 7 you can get the file exporter.
Or if you can find a program or collection of programs that will export the Collada file format, you can import those.
What I'm doing right now is create models in Rhino, exporting 3DS files from Rhino into Blender, doing my texturing in Blender then outputting Collada files from Blender. My other option was to export 3DS files from Rhino, import them into Gmax, export MDL files from Gmax to MilkShape and exporting Collada from there. If I could afford 3DSMAX, I would have gone that way. I'm still waiting for someone to make a Gmax to FreeSpace plugin. LOL
I did play with TrueSpace for a while, but I absolutely hate it.