At the end of each post on the blog there is a line of small text that looks like this:
"Posted by Prof John Garver and Prof Jaclyn Cockburn at 12:06 AM 6 comments"
All you need to do to post a comment or read the other comments is click on the word comments. It will open a new page and there you will see an option on the top right to write in your comments and read the other comments on the left. You shouldn't have to create a profile, but it might be easier if you do.