how do I create ground shadows in studio 3 ??

thanks

Remember that the floor grid in DS is just an indicator and not an object in the scene.  You need to have an actual ground plane or terrain, and at least one light set to cast shadows.

Perhaps reading through the forum posts, as well, to find answers to questions already posted?  The application came with a pdf file that is your manual as well, that may answer some questions for you!

"Remember that the floor grid in DS is just an indicator and not an object in the scene.  You need to have an actual ground plane or terrain, and at least one light set to cast shadows."

^this. also, in the settings for any light you put in the scene, try using deep shadow map (or rayrtracing if your comp can handle it) for better shadows

You can use the ground plane or any objects within the scene, but if you want to create a shadow using the shader-mixer then you should view Carnite's video on this link below..
http://www.youtube.com/watch?v=qo2OVy9tf8E

It demonstrates how a shadow can be created as an alpha and placed over almost any backdrop image. Hope this is of help.