Well, you would make a chat.gif button and upload it to your /images directory.
Then you want to edit your Header. Mine looks something like this... you can see where I worked in the code to include the chat button (and how it looks
here)
<a href="misc.php?s=$session[sessionhash]&action=faq"><img height=22 alt="F.A.Q" src="[imagesfolder]/head_faq.gif" width=98 border=0></a><a href="search.php?s=$session[sessionhash]"><img height=22 alt="Search" src="[imagesfolder]/head_search.gif" width=98 border=0></a><a href="http://parentingmiracles.com/chat/index.php3"><img height=22 alt="Chatrooms" src="[imagesfolder]/head_chat.gif" width=98 border=0></a>