Get a button for in the Editor

There’s an easy way.
Open functions.php and add this code. It works for many html entities

// got this form http://www.sycha.com/wordpress-add-hr-button-tinymce-visual-editor

function enable_more_buttons($buttons) {
  $buttons[] = 'hr';

 /*
  Repeat with any other buttons you want to add, e.g.
  $buttons[] = 'fontselect';
  $buttons[] = 'sup';
 */

 return $buttons;
}

add_filter("mce_buttons", "enable_more_buttons");
//add_filter("mce_buttons_2", "enable_more_buttons"); // add to second row
//add_filter("mce_buttons_3", "enable_more_buttons"); // add to third row