Menu callback error : Undefined offset: 4 in _menu_translate()
What is the % parameter in the url used for - a feed item maybe?

For that type of UI, you may need to add a load function and use a named parameter instead for the %

e.g:

/** 
 * Implements hook_menu 
 */
function planet_extension_menu() {
 // ...
 // ...
    $items['admin/structure/feeds_admin/manage/%pe_feed_item/delete'] = array(
    'title' =&gt; 'Delete',
    'page callback' =&gt; 'drupal_get_form',
    'page arguments' =&gt; array('delete_feed',4),
    'access arguments' =&gt; array('access administration pages'),
    'type' =&gt; MENU_LOCAL_TASK,
    'file' =&gt; 'planet_extension.admin.inc',
    );
 // ...
 // ...
}

/** 
 * Menu callback load function for pe_feed_item argument
 * This function loads the feed item 
 * which is then passed as a page argument to the menu callback function
 */
function pe_feed_item_load($fid) {
  // Load feed item somehow from feed id and return it, e.g:
  return db_query("SELECT * FROM {pe_feed_item} WHERE fid = :fid", array(':fid' =&gt; $fid))-&gt;fetchObject();
}