Joomla! Documentation™

Starting with version 2.5.4, the Joomla core is updated with the new component Joomla Update instead of in Extensions/Install/Update. If Joomla Update shows as an installed component but is not in the Admin Menu under Components, you are missing the entry in the database table #__menu.

INSERT INTO `#__menu` (`id`, `menutype`, `title`, `alias`, `note`, `path`, `link`, `type`, `published`,
 `parent_id`, `level`, `component_id`, `ordering`, `checked_out`, `checked_out_time`, `browserNav`,
 `access`, `img`, `template_style_id`, `params`, `lft`, `rgt`, `home`, `language`, `client_id`)
VALUES (22, 'menu', 'com_joomlaupdate', 'Joomla! Update', '', 'Joomla! Update',
'index.php?option=com_joomlaupdate', 'component', 0, 1, 1, 28, 0, 0,
'0000-00-00 00:00:00', 0, 0, 'class:joomlaupdate', 0, '', 41, 42, 0, '*', 1);

Replace #__menu with the name of your database table. Under VALUES you need to change the id, component_id, lft and rgt to the appropriate values for your system.