This page cannot be edited in SharePoint Designer message
I wanted to add new web part zone to a one of my publishing page in an intranet. But I was getting above message and couldn’t edit the page. The reason for this is,
This message tells you that publishing pages that are based on a page layout cannot be edited in Office SharePoint Designer 2007, and that their content can be edited only in the browser. To edit the underlying page layout from which the page was created, you can click Edit Page Layout. (Note that you are able to rename pages in SharePoint Designer. To do so, right-click the page in the Folder List, and then click Rename on the shortcut menu.)
However, if you choose, you can edit a publishing page in Office SharePoint Designer 2007 by detaching it from the page layout that it is based on. For example, you might want to customize the Search Results Web Part on a publishing page. You can do so by detaching the publishing page from the page layout, then customizing the Web Part in SharePoint Designer, and finally reattaching the publishing page to the publishing page layout. To do so, right-click the page in the Folder List, and then click Detach from Page Layout on the shortcut menu. You will see the following message.