It is a bit tricky I do agree Mailbox.
1. First of all you click on blogs (on the top menu bar)
2. Then when you have the blog post in front of you there is a "comment" link, click on it. It will bring up the blog post with comments at the bottom of it.
3. Scroll down the page to the comments. Then click inside the empty comment box.
4. You'll see there is a request for you to sign in and it offers a few choices including "register with Disqus"
5. Click on "Register with Disqus"
6. Complete the three boxes - user name, e-mail address and password and click on the arrow next to it for "OK"
7. You're ready to make your comment
8. Next time round click on the D logo to make a comment