PG,
When you open the box,and see the drop down,if you look across the very bottom of the box you will see a narrow portion across the bottom.You need to hover your mouse over this area,and click,you will then see a vertical cursor and can begin to type your message.When you have finished typing,press "Enter" on your keyboard,and the message is sent.
Couple things: It is easiest to keep the box open all the time,then you dont have to put up with the annoying flashing "message waiting" box mentioned by priestess.If you choose to type something as the last shot of the hole is played,the load box for the next hole pops up and takes priority,so whatever you type will not show,and you have to type it again.
Each time you use your mouse to take a shot, it disables the typing cursor,so you then have to go back to the step mentioned above in order to set up the chat facility.
If you wish to say thankyou for the game at the end of a round,you need to do this before the last shot is played,because the chat facility is disabled immediately.
You should endeavour to type your message either in your own allotted time for shot,or after a shot has been played and before the game progresses to another players shot,so that you do not inadvertantly have your sent message interfering with anothers players concentration.
Use chat sparingly if on a 60 or 30 second timer.
I hope this helps.
Lizzie