Menus and paths are slightly different things...in `hook_menu()` you're defining paths that can be consumed; menus create the links to those paths - the two concepts aren't necessarily related (a path can exist without a menu link).

You can assign a particular router path to a menu using the `menu_name` property in your `hook_menu()` items, but that menu needs to exist already. 

Fortunately Drupal provides a method for adding links to the top of a page through `hook_menu()`, and you only need to change one line:

    $items['admin/custom/add'] = array(
      'title' => t('Add Custom'),
      'decription' => t('Add custom'),
      'page callback' => 'custom_add',
      'access arguments' => array('add custom'),
      'type' => MENU_LOCAL_ACTION, // <-- This one changed
      'file' => 'custom_add.inc',
      'file path' => drupal_get_path('module', 'custom') . '/includes',
    );

That will put a link to 'Add Custom' at the top of the 'admin/custom' page; if you're using the Seven theme for administration it'll look like the 'Add Content' link on the content admin page:

![enter image description here][1]


  [1]: https://i.sstatic.net/vZXoD.png