Form ids change in Drupal 7.
comment_node_*_form where * is the content type. The id for a story is
Use Firebug in Firefox. Select a spot in the form where you want the captcha. Right click and select
Inspect element. You will see the form element with an id of the form
comment-node-page-form. Replace the dashes with underscores to get the id required in Captcha.
You can also switch on
Add CAPTCHA administration links to forms in the Captcha administration page then visit a page of each content type, add a comment, then switch on the captcha in the comment page.