Click on the white text field at the top. This will open a dropdown to login. You can log in with your exsiting social media email by clicking one of the social media icons. You can instead create a login with Disqus. If you already use Disqus, just click the "D" icon. After you type or paste in your comment, click "Post As...."
You can post as a guest without loging in. Click on the Join the Discussion field (where you post your comment). Then click in the "Name" field. A "I'd rather post as a guest" checkbox will appear. Check it. Guest commenters do not receive email notifications and do not have profiles showing their comment history.
Click on "Reply" at the bottom left of a comment.
You can choose how you would like to list comments using the "Sort by" drop-down menu. You can list comments by best, newest or oldest. If there are more comments than allowed on one page, you will see a page select link at the bottom to view more comments.
You can vote on comments by using the up or down arrow at the bottom left of a comment. Votes affect the order of comments when you choose to view by "best" under the Discussion drop-down menu. This allows you to see the most popular comments first.
You can subscribe to threads to be notified when there are new posts made. Click on the mail icon at the bottom left of the Community Board.
You can share a thread on Facebook or Twitter. Click "Share" under any post.
The Community link next to the Discussion link will list people who have commented anywhere on the web site. Clicking on person will list their comments and where they are posted on the web site.
More about Disqus: http://disqus.com/