To use this feature, go to the sidebar, scroll down to the "Multi Media" section, add your name, website url (if applicable), and message, then click "Tag". That's it! Your message will be instantly visible.
For now, you may see a load of pop-ups (unless you are using Firefox as your browser as I do; it's pop-up blocker works quite well). This should subside as soon as Tagboard finishes processing my order for an upgraded (read: paid) account; I ponied up the $20 for the Pro account to kill the ads.