As for all of Manhattan's modules, the teacher can control whether or not the Chat module is available within the classroom. Activating the Chat module is done the same way as for the other modules. Start from the Main Menu of your classroom (that's the page you see as soon as you enter the classroom), and click on the Configuration button at the bottom. (If you don't see a Configuration button, then you are not logged in as the teacher of the classroom.) The first item in the configuration menu reads:
Go here to turn modules on and off. You can also change the course information that appears on various screens throughout the system.
Clicking that hyperlink will bring you to the form where you can activate the Chat module. Chat is listed near the end of the form. Scroll down, click the "Enable this module" box next to Chat, and click the Update Configuration button.
Once you've activated the Chat module, a Chat button will appear on the Main Menu for all members of your class.