Your problem is in the Compositor - you don't have your render layer hooked up to a Composite output node, in fact I didn't find any nodes when I switched on 'Us Nodes' so I rigged up a simple set up. You scene didn't have enough light so I turned on AO, and I set your Shading tab to use alpha instead of render sky. I hooked the render layer alpha to the mix factor of the color mix node, and plugged the image node into the other color as a background(this is wher eyour video footage goes). The file should help a little to fix the original file hopefully. redone file

Your problem is in the Compositor - you don't have your render layer hooked up to a Composite output node, in fact I didn't find any nodes when I switched on 'Use Nodes' so I rigged up a simple set up. Your scene didn't have enough light so I turned on AO, and I set your Shading tab to use alpha instead of render sky. I hooked the render layer alpha to the mix factor of the color mix node, and plugged the image node into the other color as a background(this is wher eyour video footage goes). The file should help a little to fix the original file hopefully. redone file