Its easy Countess Richard.

You click on the drop down icon to the right of where it says quick links at the top of the mudcat webpage.

You scroll down and select the menu choice Mudchat ( ) and then click on Go

You then click on the button that says Enter Mudchat.
(Little point if you can't see anybody listed under Active Chat Users)

You then type in the message box at the bottom of your screen and hit the Enter key on your keyboard. Your message will be displayed and others will respond. etc etc.