Hi dk,
When you're writing a reply to a post, click the "attachments and other options" underneath the text box, and click "Notify me of replies". I'm not sure how to do it without putting at least one reply in yourself (there may well be a way).
However, please note that at the moment, the forum is not able to send ANY emails due to the untimely death of the e-mail server... until I can get the DNS records changed to a new server, this won't be changing. Annoyingly, the domain details were stored in an e-mail on the old server, which has karked it....
Rest assured, as soon as I can I'll have it sending e-mails again, watch the news items at the top of the page for more details.