You need to make the terrain an entity unless you use Rolfy's method but that will require a full 3D modeller as you need to slice your terrain mesh up and put it back together in the FPSC segment editor.
As Geoscape3D is a dedicated terrain modeller you will need to make it a static entity by creating a .fpe file for your model and texture.
You could potentially make it a segment but only if the terrain model was quite small and didn't crash the segment editor trying to import it. Rolfy's method is the only way to make large terrain segments properly.
For a working example large scale terrain including .fpe file which you can use as a template. Download the terrain I prepared and released a while back from the following link:
http://forum.thegamecreators.com/?m=forum_view&t=100284&b=24
Screenshot attached
Enjoy!