How to post comments?

Ok, so I was trying to reply to the comments, and it won't let me! I click on sign in in the right hand top corner, and it takes me to my home page where I can write posts and stuff. But then when I click on "view blog" to post a comment, I can see the blog and the comments, write my own comment, and then when I click "publish" nothing happens. I don't know if its signing me out every time I click on "view blog" or what. I also tried to comment on another blog (Kristin's) and the same thing happened. How did you guys do it?