I purchased a video clip from istock, the video clip is an .mov and has a alpha channel included. The video clip is santa clause over a black background and tagged at the end is a white solid of the video (I assume this is the alpha channel) How do I use the alpha channel to help key out the vidoe clip that I need. here is a link to the actual video clip I purchased. http://www.istockphoto.com/stock-video-11011749-santa-claus-with-alpha-channel-ntsc.php
As Szalam said, the b/w image can be used as a luma matte.
Also check if the actual piece of footage has an embedded alpha channel, i.e when you import it AE should know there is an alpha channel and let you know in the top of the project window when you select the element (or "interpret footage").
Alternatively just drag the footage to a comp and enable the checkerboard background to see if there is transparency. These three approaches are the same...just different ways of finding out about the alpha channel.