Just follow the steps listed in this blog post.
1. Go to "My Account"
2. Click the Profile Tab
3. Click the link that says "Acquire/Refresh a Live Messenger Permission Token" (do this again even if you did it already)
4. When you are redirected back to My Account, click the Profile again and chweck the box for "Enable Live Messenger" and click the Update button
Now find your name on the member list, click the View Profile link to see your public profile and the chat will appear. If you sign in to Live Messenger it will show your status and users will be able to chat with you.
To enable it on your own site you may also need to get a Windows Live App Id and secret key and enter it in Site Settings under the Security > Widows Live Tab
Hope it helps,