Re: Alpha channel mask (easy one this!)

Put the layer with the alpha channel above the other layer and choose on the layer beneath the alpha-channel-layer Track Matte>Alpha Trackmatte (or inverted if you want to)(its all in the timeline)

This Method effects only a single layer.

For effecting all layers underneath your alpha-channel-layer change its blending mode to>Stencil Alpha (or one of the other 3)

And if you want a mask for a layer:

Select your aplha-channel-layer (i love this words :-) ) and choose layer>Auto Trac. After changing a few settings in the upcoming menu(dont worry you even dont need to change) click apply and a mask will appear on a new solid.
Now you can copy the mask to your layer that should have the transparency from the...one more time: alpha-channel-layer.

Hope this helped