How do I start a conversation with someone using Facebook Chat?
You can see which friends are online by clicking the Chat menu at the...
You can see which friends are online by clicking the Chat menu at the bottom right of any Facebook page. A green dot will appear next to a friend who is online and available to chat, and a moon will appear next to a friend who is currently idle. On profiles and anywhere else on the site, clicking on a chat icon will start a conversation with that person.
To send a Chat message, simply type your message and then press "Enter" or "Return" on your computer keyboard.
Please note you can only message with your confirmed friends using Facebook Chat.
if this doens't help here is the link to chat help on fb
http://www.facebook.com/help/?page=713