Hi there. I can already see some volumetric shadows coming from the clouds. It's subtle but they are there.
A couple of pointers to get volumetric shadows,or "God rays".
Firstly, the shadows or rays need a medium to work in. If you set your haze density to zero you would see no rays. The default is 1, maybe bring it up to 5 and see how that looks.
Secondly, you will always get more visible shadows/rays if the sun is in front of the camera, even more so if the sun is "in shot" ie within the camera FOV. If the sun is to the left or right of the camera or behind the camera the shadow/god ray effect will be greatly diminished.
Thirdly, if the camera's position is illuminated by the sun then this also diminishes the shadow/god ray effect. Try to place the camera just inside a cloud shadow, with the area of interest illuminated by the god ray.
And finally ... Terragen calculates volumetric lighting effects automatically,so it's not so much about tweaking sliders and settings but rather about placing the sun and clouds in such a place that will automatically create a god ray/volumetric shadow because under those circumstances in the real world that is what you would see.
Hope this helps
Richard