You can put a chat box in your Header, your Footer, Portal Block or a Board Description area

To do it in your Header or Footer go to:  Admin > Manage Styles > Edit Header/Footer

To do it in a board description create a board for the shoutbox,  paste the shoutbox code in the boards description area.     Its nice to do it this way as you can limit who can see it by the boards permissions

If you are using the Portal System and want it on your portal, make an html block and put the code in there.
Complete Guide to Setting up the Portal
   

You will need to sign up for a shoutbox/chatbox someplace and get the code:

     Here is a list of chat/shout boxes:
     
     www.cbox.ws
     www.yellbox.com
     www.myshoutbox.com
     www.shoutmix.com
     www.shoutbox.us
     www.tag-board.com
     www.tagboards.tk
     www.tagboardsite.com/
     www.xat.com/