Adding the current node title as the value of the comment form title?
You're right, you most likely have the $form_id wrong.  To check this you can use drupal_set_message($form_id); and refresh the page to see all form ids on the current page.  

Once you have the form id correct, then you can use print_r($form);

For example, I got this working on the comment form on my node type "Announcement":

function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'comment_node_announcement_form') {
    $form['subject']['#default_value'] = $form['#node']-&gt;title;
    //print_r($form);
  }
  //drupal_set_message($form_id); // Use to get $form_id
}