[JKT]

James, if you select a message title and use the shortcut, it will create a new item in your Inbox/Quick entry field, with the contents of the message appended as a note.

As I wrote before, I don't think it is meant to create a new toolbar button in Mail - you are just meant to use the keyboard shortcut.

Incidentally, it might be less than trivial to create a toolbar button for Mail as, depending on what you are looking at, you can have different toolbars - e.g. the toolbar set-up for the main window is different to that for individual messages (when viewed in a separate window, rather in the preview pane).