cakePHP basic ajax form problem

i am trying to do a simple form that adds a new comment using ajax to a blog post (actually this is a part of a cakePHP tutorial)

but the problem is that the submit button do nothing at all

here is the part of code that generates the form in the view.ctp file

<?php echo $ajax->form('/comments/add', 'post', array('url' => '/comments/add', 'update' => 'PostsComments', 'indicator' => 'commentSaved'));?>
<?php __('Add Comment');?> <?php echo $form->hidden('Comment.post_id', array('value' => $post['Post']['id'])); echo $form->input(''); echo $form->input('Comment.content'); ?>
<?php echo $form->end('Submit');?>

and here is the add comment action in the comments_controller.php

if (!empty($this->data)) {
    if ($this->Comment->save($this->data)) {
        $comments = $this->Comment->find('all',array('conditions'=>array('post_id'=>$this->data['Comment']['post_id']),'recursive'=>-1));
    } else {

the problem is that the action add isnt called from the view ... when i viewed the generated html source code i found something like that

if i removed the on submit tag manually then the action is called but the add_success.ctp is generated as a new page not as an ajax call.

so what could be the problem ?