From http://www.sycha.com/wordpress-add-hr-button-tinymce-visual-editor Add this to functions.php

Wordpress add hr button to wysiwyg.php

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");