hmmmm letseee....
in premiere pro 1.5 you choose the clip you want, and then go to the effects bar and go down to video effects,
then to the group that says "keying"
in there they have a bunch of different things to choose like "green screen key" or "blue screen key", or the "chroma key"--which you can then go into the effect controls for it and choose exactly what color you want to remove from the clip.
Edit: Also if you go to the effect controls you can change the opacity to a different percent if you just want to make the entire clip transparent, but that wont look as good as keying out a background.
but yeah, they refer to it as "keying" in premiere, but im not exactly sure where they have it in elements, so if you can't find it you might be able to try searching for that in help to see what comes up.