Quick Note: the Administration Menu module in Drupal seems to have some weird bug in ver. 5.2.5 which causes menu items listing to disappear on admin/build/menu configuration page.
Solution: disable and remove Administration Menu module, download 5.2.6 or later version, install that one. If the problem persists run “menu_rebuild();” from hook_init() of any module or even from page.tpl.php. The problem should be gone.