In an attempt to minimize the dreaded spam, you will need to register as a user. It is free, but requires a bit of navigating. Once it's done, though, it will be easy to comment.
At the bottom of each post, on the right side, is the word "comment". If you click on this, a new page will appear with instructions for registering for a Google Blogger account.
You will be asked for a valid email address, to create a password, and a display name. The display name is the name that will appear when you leave a comment. You can use your real name or not.
You will also be asked to verify letters (spam eliminator) and if you accept the terms of service. Then you press continue. At this point, you will have the opportunity to create your own blog. If you don't want to do this, just close your browser. Go back to the blog and sign in...and voila...you can comment all you like.