You see Script-fu in the menu bar if some script you load tells Gimp that its menu is in <Image>/Script-fu. Only very old scripts do that. In ancient versions of Gimp (2.2, 2.4 at best), there used to be a Script-fu menu for all the add-on scripts. In later Gimp versions scripts can be added in more appropriate menus and the catch-all menu is Filters.
If the Script-fu entry is not created, it may be because your script is so old that it has become incompatible with recent Gimp versions and fails during registration. If you tell us what it is or what it does we could point at possible replacements.
If the Script-fu entry is not created, it may be because your script is so old that it has become incompatible with recent Gimp versions and fails during registration. If you tell us what it is or what it does we could point at possible replacements.