If the object contains bryce primitives, booleans, etc. (anything besides terrains and symetrical lattices) it can't be exported. If it is a terrain, symetrical lattice or an imported object (.3ds, .obj, etc with none of the things mentioned above that can't be exported added to it) then click the little e next to the object when it's selected which brings up the smoothing screen then hit ctrl-d and this brings up the export screen.