I've had some friends and family say that they've had trouble commenting.
I just went through the process myself and am happy to report it work! Even using the new (awesome) browser, Google Chrome.
Here's how:
Select "Comment on this post" or "[#] Comments" at the bottom of the post.
Type in the text box provided.
"Comment as" should be filled in with either your Google ID, Anonymous, or another alias. If you fear giving out information online, you may comment anonymously, but please leave your name in the text so I know who you are!
Then click "publish".
You will be redirected to a similar screen that asks you to type in the characters for a letter/number validation. This is a built in safety feature that keeps all comments authentic.
Then click "publish" again.
See your comment? Great!
Don't see your comment? Let me know... I'll try to help you figure it out.