Very easy, you just have to create a groundplane!

By default there is no groundplane, any figure is loaded at y-position 0, so if you want shadows go to "Create->primitives->plane" you may scale it as you need and hit render.

Another thing, make sure you are using the 3Delight renderengine and not the OpenGL render!

If you want to composite your render with a photo, have a look over at DAZ at PoseWork's pwCatch, correctely applied to the plane, D|S will only render your character plus the shadow but not the rest of the plane, very handy!

Pose over a white background to see all details but render over a black background and save it as .png, so you get the alpha-information integrated.

Peter