These two are a little bit tricky and we should probably be explaining them better :-)
You can read in detail about all the Flash Player blend modes here: http://help.adobe.com/en_US/Flash/10.0_UsingFlash/WSd60f23110762d6b883b18f10cb1fe1af6-7d5c a.html
The short version is that Erase and Alpha only work when you place them inside a component, and then change the overall component's blend mode to Layer. Once you do that, these blend modes function like a "knock out" -- one of them copies its alpha to the overall component's alpha, and the other copes the reverse of its alpha.
That's probably still a little confusing, so I've attached a picture. Both squares just have solid border and a fill that is a gradient fading black to transparent. They are inside a custom component, the big blue rectangle. You can see the Alpha square is making the blue rectangle transparent everywhere the square is transparent; and the Erase square is making the blue rectangle transparent everywhere the square is opaque.
One other thing to note: you won't see the effect of these blend modes while you're editing the custom component. But once your leave the "edit in place" mode, you'll see the knock-out being applied.
alpha-erase on.png 4.4 K
I have a 45 second green screen .MOV I have imported into Adobe After Effects. I am rendering the file as a .flv with Channels: RGB + Alpha Depth: Millions of Colors + Color: Premultiplied. I have brought the clip into Catalyst and created a custom component out of the video. The custom componet is set to layer blend and the sub componet video player is set to alpha. When I have this setup, I get sound with no video whatsoever. I have checked After Effects Alpha is set correctly to the person speaking. What am I doing incorrectly?
Does it work if you set the video player's blend mode to something simpler like Normal or Difference? If the video does appear in those blend modes, it is transparent in the places you expect it to be?
Also, are you testing this by playing the video within Catalyst, or are you running it in a browser?