First, find your Facebook username by going to https://www.facebook.com/your_user/. Next, Open iChat, then select iChat » Preferences and click on the Accounts tab.

Click on the + (plus) sign to add a new account, with these settings:

• Account Type is Jabber Account
  Account name is your_usersame@chat.facebook.com, and enter your password
  Click the drop-down arrow to reveal Server options. Enter chat.facebook.com as the server name.
  Enter 5222 as the port, don't check SSL and click Done.

Click Done again, and you are good to go.