Render Layer Transparent

I’m using this tutorial http://cgcookie.com/blender/2011/12/07/blender-overview-of-object-tracking-in-tomato-branch/

when I render, the background layer becomes transparent. heres what it look like when I render it. http://www.flickr.com/photos/[email protected]/7033787609/sizes/l/in/photostream/

also, I downloaded a plane from blender swap for the background, and only the shadow will render. thanks:o

when I render, the background layer becomes transparent. heres what it look like when I render it. http://www.flickr.com/photos/7564146...n/photostream/
The image shows as Private when I go to the link. Just use the image attach function when writing your post, much easier that having to go to another site just to see the image, assuming others users are allowed to see it.