Navigation Lock isn't on the default context menu. You can add it to any menu, toolbar, context menu or hotkey using this command:
Set NAVLOCK=ToggleA pre-made Navigation Lock button which runs that command can be found in Settings -> Customize -> Commands -> Go and you can drag that to a toolbar or context menu.
If you want to edit your lister context menu, go to the Context Menus tab in the Customize window, select Lister Context and click Show; then edit it like any other toolbar or menu. (See the Toolbars 1 video in the tutorials section of the forum to learn the basics of toolbar editing.)