From their website info, it looks like you need to upgrade from the free version to at least the "Plus" version to import/export 3DS format. That's $34.95
Another alternative would be to use a separate application to convert from one format to 3DS or back again, like Unwrap3D or Milkshape3D.
Another alternative would be to use a separate application to convert from one format to 3DS or back again, like Unwrap3D or Milkshape3D.