Did you interpret the footage with the proper alpha channel? Look in AE Help under interpret footage, or use the AE Help search function.
Does it even POSSESS and alpha channel at the moment? Put the footage in its own comp -- nothing else in it -- then look at the switches at the bottom of the comp window. You can turn on the familiar Photoshop checkerboard which indicates transparency, or you can use the three-color switch to view the alpha channel.
Did you also need to get the Matte footage for this video? You'd better check that, too.
So there's your assignment for the day.
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA