See http://support.microsoft.com/?kbid=284896 "'Most recently used' list
missing from File menu and 'Recently used file list' option unavailable
in Office XP and 2003".

Michael O. wrote:

I tried opening the general options under the tools command but the "Reveal
used file list" command is grayed out and "un-clickable", does anybody have a
solution to fix this? I tried to detect and fix the solution and I set back
to the original settings but that hasn't helped.