There are two ways.
1. Click on the green Title of the post, this will open that post up in a "single post" view and show all of the previous comments and the comments box will also appear so you can leave your own comments on the post.
2. The other way, which does the same thing. You will see at the bottom of each post "0 Comments" or "5 Comments" Click on this and it will open the post in single post view and all of the previous comments will show up including the comments box that you can use to leave your comments.