Hi Gerard,I don't personally use the masked textbox so couldn't help you much with how the masking work's, I use Dev Express controls which have really nice masking features built into them so I never have used the standard masked text box I just knew it was there in the toolbox.
Since the masked textbox is a just a standard winforms control I'm sure there should be info available either in msdn documentation or via a google search on how to use that control.
Paul