I have a PS shape layer, a snowflake and when I save it and import to AE I want to make a mask around it. The tut Im watching they have a mask following the edge of the flake. But when I make the mask it just makes a square round the whole thing. I also noticed when saving that alpha channels is greyed out in PS save dialogue.
Did you import your Photoshop file as footage or as a composition? Import it as a composition and see if that helps.