Is there only one layer in the composition you're working in? Is your footage only in the composition ONCE? If you have the masked footage on a layer that's directly above another layer that contains identical footage (albeit unmasked), then your mask is not going to show up like you want it to. When the mask on the upper layer is completed, there will be no black visible because the second layer of footage is filling in the area that would otherwise be black/hidden. You'll still see the outlines of the mask if you have it selected, but otherwise it will seem like it's not actually masking anything.
If you don't have the above situation, then can you post a screenshot of your work area with the lines of the mask visible?
And if you need help with the pen tool in AE, I can give you some directions. It's different enough from Photoshop (not to mention other Adobe programs) that figuring out its quirks can be a pain in the butt if you're not really comfortable with it to begin with.