Hi MikeJRW,
For a 3D scene, you have a few variables that can be moved in order to composite the 3D elements into a 2D background image. There's the "Ground Plane" which defines a flat surface where shadows and reflections display, there's lights which can control the direction and size of shadows, there's the object's location in space (not always resting on the Ground Plane), and then there's the camera view and attributes which defines perspective and field of view.
Most basically, you want to adjust the camera view (click on the canvas with the Move tool selected, until you see an orange outline around the document view) in order to move the ground plane around relative to what you're viewing of the background image. There are options for how the camera moves in the tool options bar, upper far right.
To get everything aligned so that the composite looks realistic can be tricky and time consuming. In Ps CS6, we've added a few things to make it easier then in earlier versions. The first thing you might look at trying to do is defining the background image's horizon and perspective using the Vanishing Point filter (search Vanishing Point filter for usage instructions). Now, assuming you have a Ps generated 3D extrusion from a vector path, export that layer out as a Collada (.dae) file, so it can be imported into the background image file. To import the 3D mesh file go to 3D > New 3D Layer From File... and the 3D mesh should align much better with the perspective of the background image. You might need to adjust the scale of the 3D object.
You also want to adjust the lighting direction to match that of the background image, so the shadows are in the same direction and length. You can do this by selecting the lights in the 3D panel to get a light control widget showing on-screen, or by Shift+ click directly on the shadow and then dragging it around.
I did a real quick overview here, and there are many step by step tutorials that go into more detail. I hope this helped.
regards,
steve
