Can you clarify what you mean by 'selecting the command' and 'nothing
happens'?
IIRC in Word 2000 'Track Changes' is a submenu - are you saying the submenu
items are missing (which might be due to customizing the menus) or that the
'Accept or Reject Changes' item is greyed out (which might be due to having
a protected document, perhaps?)
If you are actually in Word 2002 or 2003, then Tools > Track Changes is a
command that toggles revision tracking on and off (so if you select it from
the menu you won't see any immediate effect until you start making changes).
In these versions you need to use the Reviewing toolbar to accept or reject
changes.