If you install linktocontent and linktocontent_node on a multi-site installation 
of drupal or the file plugin_reg.php (in your tinymce directory) isn't writable 
for any reason, you have to copy the following line into plugin_reg.php right 
before the last lines (starting with <code>return $plugins;</code>). 

// linktocontent.module: linktonode
if (is_dir(drupal_get_path('module', 'tinymce') . '/tinymce/jscripts/tiny_mce/plugins/linktonode/')) {
  $plugins['linktonode'] = array();
  $plugins['linktonode']['theme_advanced_buttons3'] = array('linktonode');
}
