Once I create an alpha channel, how do I edit it? I have tried selecting and clicking the alpha channel, and then using the paintbrush tool. I understand that if I have a white foreground, painting should remove some of the mask, while a black background should result in the painting adding to the mask. But often nothing seems to happen when I paint. Or, at other times, the painting results in paint marks on my image but doesn't affect the "marching ants" and therefore doesn't affect the mask. I have already checked that my K setting in Color is at 0 for white and 100 for black. Using CS5 version 13.0 x64 with Windows 7.
Apologies for the newbie question. I must be missing something very obvious. I have googled this and not found anything helpful. (Perhaps I am searching on the wrong words.)
One thing i see is you actually have your selection of the sky channels active, so you should really not have the selection active when painting on the channels.
Selections (marching ants) aren't changed in that way.
Try Select>Deselect and then paint on the channels.
Then you can Ctrl (Cmd) click on the channel thumbnail in the channels panel to make a new marching ants selection that reflects your painting of the channel.
(or click on Load Channel as selection at the bottom of the channels panel)
An Alpha channel is a greyscale image. You can do whatever you want: paint on it, do a Levels adjustment, a gradient, a filter (Gaussian Blur for instance), etc.
But make sure you have no selection active on that channel as you cannot make any change outside the selection.
Also make sure your brush is in Normal mode (or a mode that has effect on what you are painting, e.g. not Overlay mode on white or black paint, although that mode is very useful for adjusting channels and masks but that's another story...)
Once you are finished with working on that channel you can reload it as a selection of course...