Select any user and write a message. On the page of your blog, all sections on the left side, one of them - "communication". In this section you will see your first message, and later this will display all other messages. Expand any correspondence by clicking on the text (if you click on the nickname of the interlocutor, the user's blog opens).
A mini-chat icon is displayed on every page in the lower right corner (except the site home page and the "communication" section). The interface is very clear.