For Excel XP(2002) on Windows XP:
C:\Documents and Settings\UserName\Application Data\Microsoft\AddIns
(Change UserName to your Windows username)
Note, the Application Directory may not appear in Explorer if it is setup to not show hidden files and folders. In this case, type the full path in as it appears.
Excel 2000 on Windows 98:
Excel 97 on Windows 98:
C:\Program Files\Microsoft Office\Office\Library