If you hover your mouse over the word 'comments' you should be able to click on the hyperlink to the comments section. It will pop up in another screen, showing all other comments, and a blank comments box at the bottom for you to type in. Type your message as desired.
Sometimes there will be a word verification thing next on the screen where you have to copy the letters/numbers listed to verify that you are a human leaving a comment, and not a computer spamming me. This is just a security feature. Just copy those letters, then leave your details in the name, email etc boxes...so I know who you are!