If your already logged into MACCAforums then all you have to do is click the CHAT link along the top.
If you have java it will log you into the chat room with your MACCAforums nickname, it may take a minute the 1st time as it will download a bit of java.
If this doesn't happen then you may not have the java plugin for your browser installed, see the links at the bottom of the chat page.
This link will test your browser and if it tests ok, you'll see a dancing logo, it will tell you what to do. This should help you.