In my experience using PageMill 3.0 on a Mac, this message usually means that you have a locked file on your site. To resolve this on a Mac, use the "Get Info" command (Apple + I) on each suspected file until you isolate the one(s) locked. A locked file will have a check in the small box in the lower left-hand corner of the Get Info window. Uncheck to unlock. Otherwise, I have heard that PDF files will also cause the same error message.