At the bottom of a blog post you will see this:
If no one else has posted a comment on that blog post yet it will have 'No Comments:' in red. Click on 'No Comments:'. The page will then re-load with a box like this:
Type in your comment in the box like this:
Click on 'Publish'. The page will re-load again and show your comment, like this:
Adding more comments or replying to comments is the same process. Type in the empty box, click on Publish.