Scroll to the bottom of the specific entry and you should see the 'Comment Box'. Simply write your message in here and click 'Add Comment'. Your comment should appear at the bottom of the comments as they read in order of date.
You can emphasise all or part of your comment with the edit tools in the bottom left of the entry box (this is available when using a browser on a desktop or laptop computer). Just highlight the letter(s), words etc and click on the edit icon you want to use.
You can also add a hyperlink using the external link icon. This will open a new window to add the URL. For new members this option isn't available until you've reached your first milestone.
If you've highlighted a word or phrase to use for your link there is no need to add a description.