You are aware that if you had searched you probably would have found your answer.
At any rate FPSC uses DirectX format models only.No other file format will work.
You have to convert your .3ds models to .x and there is a converter available for that if you search for it.
On the FPSC downloads page (click on
The Game Creators at the top go to the FPS Creator page and then to the downloads page)
You will find the Make Entities from X Files application.Download it.It comes with a video tutorial to help you use it.
After you watch the tutorial you should have a pretty good idea on how to import your models.