I downloaded the FW demo a couple months ago. To use a pre-recorded video, I found a virtual webcam application. There are actually quite a few if you google it.
Rampa is right, there are other options available if you want to use video.
Although you can't load a video file directly (MP4, AVI, WMV etc.), you can even cheat by pointing your camera towards a video playing back and grab mocap from that.