
Are you certain your camera is pointed at the particles? Is there an ImagePlane in the camera that is occluding them? You don't mention a light; is Lighting turned off in your Renderer3D?
Typically, you wouldn't put your MediaIn directly into the camera unless you need to use the footage as an image plane. A more usual workflow would be to Merge the output of the Renderer3D over the footage. That way you'd get independent control over blur, color correction, etc of your particles.