To comment on someone's track, you will first need to be logged into eMastered. If you are not logged in, you can sign in by clicking the top right "login" button, or if you have not previously made an account, you can click the "sign up" button also located in the top right corner of the website:
To comment on someone's song directly, scroll down to the bottom of the page, and look for the comment box:
Write in the comment you'd like, and click the red "Send" button to the right of the comment text field. From there, the comment will be published on the library page.