WordPress.org

How can I tell if it's working?

1. Log out of WordPress
2. Leave a comment on an entry and check the comment subscription box, using an e-mail that is NOT the WP admin e-mail address or the e-mail address of the author of the post.
3. Leave a second comment using a different e-mail address than the one you used in step 2 (it can be a bogus address).
4. This should trigger a notification to the first address you used.

I'd like the subscription checkbox to be checked by default. Can I do that?

Not anymore. But the checkbox status will be remembered on a per-user basis.

My subscription checkbox shows up in a strange place. How do I fix it?

Try unchecking the CSS "clear" option. Beyond that, you're on your own with CSS positioning.