You need to load adobe flash.  Chat is flash based.  
http://get.adobe.com/flashplayer/?promoid=BUIGP
As Mokoa said, most apple users are SOL.  Some version of the OS for some laptop and desktops can run flash.   I am mactarded so I can't ell you more than that about Macs.   
Will one of you Mac experts chime in on which versions of the OS or how to hack the OS to run flash?  I have read that some versions can run it.  Don't know which ones.
I am thinking the latest OS does.  One of my lady friends here has gone mac and I know she was in chat right out of the box, unless some macgenius helped her out.