Why is my alpha channel all white?

Hi... new to Shake, but have been watching lots of tutorials.

I'm trying to key out a green screen, but when I remove the green, it takes the green out of the whole image, so the image becomes purple. To solve this (correct me if I'm wrong), I need to make a matte of the stuff that's not the green screen (the actors, etc).

The problem is, to do this, I think I need to adjust the alpha channel (I hope this is right), but when I click on the Alpha channel button in the viewer, my footage goes all white.

So... firstly, why is my alpha channel all white?

(I've searched Google and this forum, and can't find an answer -- or at least not one I understand.)

I've gone into FCP and looked at the clip's item properties, and under Alpha it says "None/Ignore". I've tried changing it to "Straight" and to "White" but neither has any effect.

What am I doing wrong? And why can't I find any tutorials that really go this basic question? Hmm.

And secondly, am I missing something in the green screen process? I thought it should be simple -- drop in the footage, add the keyer node, create a matte of what's left (from the alpha channel), and then smooth out the edges. No?

Any help is HUGELY appreciated!!