"include a transparent plane with a soft shadow texture below the object in your 3D asset."
First let me explain why you are not getting a shadow.
When you export a object you are never going to get any of the information that comes from blender's renders (cycles or blender internal). In this case the shadow.
What is a shadow? You did not model it (and information in the model is all that you can export.) A shadow is a not a "thing" like your couch, it is not even the plane under it. A shadow is something the render engine calculates whenever some geometry occludes a light source. (Essentially it is shading the render engine adds)
Think about it your couch looks very different in cycles then it does exported and displayed in scene kit, and a shadow is just something created from the render engine.
So unless you exported your all light(s) and the settings, and scene kit had a raytracer that rendered just like cycles, then you are not going to get a shadow.
This is where baking comes in. You take all that lovely data that cycles wraps your model in, and "bake" it to a texture that can be used elsewhere.
It does not matter if you are baking the full color (you would do that for your couch) or just the shadows, the process is the same.
Start by UV unwrapping the object that is going to be baked (a simple lightmap pack is sufficient.)
Create a new image in the "UV Image Editor", for VR keep it small 512x512 or 1024x1024.
Now in the material nodes for that object (the shadow catcher plane) add a Image Texture node, and set it to the image your just created (make sure that node is the active node).
- In the Properties window, Render tab scroll down to the Bake section. To bake the shadows set the Bake Type to "Shadow". Now hit the big Bake button.
So to go make to the quote from the scene kit documentation:
"include a transparent plane with a soft shadow texture."
This is what they mean, somehow make an image with the shadow on it and then texture a plane with that shadow image.
Now to get this to work, you are probably going to need to invert the image because you want to drive the transparency of the plane. (Full transparent where no shadow, opaque where the shadow is.)