I remember there was a keyboard shortcut for that, but after all these years I forgot what it was. I also seem to remember that the Help menu included a list of shortcuts, and most probably a whole topic on page numbers. Have you looked there? Unfortunatelly, I no longer keep a working copy of PM in any of my machines.
Control + Alt + P (Win)
Command + Option + P (Mac)
Sometimes other programmes hi-jack Ctrl+Alt+P, e.g. MS Publisher; the alternative is Ctrl+Shift+3.