This page will tell you about adding commentaries. It hasn't been written yet, but some of the information is in the page about creating texts and, if you feel adventurous, you can just try it out anyhow - use the 'create content' item from your menu. Like with texts, you have to be an authenticated user to create a commentary .
One thing to watch out for is that you have to write a commentary about something. You can either comment a text that's already on the system, or you can create your own text entry (of course, avoiding duplication if you can). But you can't create the commentary before the text, at least not the way the site is built at present.