Drupal 7 - How to get AJAX response displayed in a ctools modal? I want to create a ctools modal popup when a button is pressed.

function custom_form( $form, &amp;form_state ) {
  $form['btn'] = array(
    '#type' =&gt; 'button',
    '#value' =&gt; t('add'),
    '#executes_submit_callback' =&gt; FALSE,
    '#ajax' =&gt; array(
      'callback' =&gt; 'custom_callback',
    ),
  );
}

function custom_callback() {
  ctools_include('ajax');
  ctools_include('modal');

  ctools_modal_render( 'Error', 'Error Message');
}


I only get the alert message with the un-rendered ajax response. How can get I this response displayed in rendered format in modal?
Checkout Documentation on: http://api.drupal.org/api/drupal/includes!ajax.inc/7
or: http://drupal.org/node/1635198#comment-6120546 on how to render drupal messages.

For more Information about ctools modal you may check: Example to display webforms (webform.module) within CTools modal windows 

Or what about a tutorial like: "Ajax modal windows, the easy way"