Choose "Play by IM: Host". You will receive a random number from 0 to
100. IM this number to your opponent.
Your opponent chooses "Play by IM: Join", and inputs the number you
gave him, and then hits the "GO" button. This number makes the starfield
identical on both of your computers.
Your opponent then takes the first turn. At the end of his turn, he
will see a set of three numbers beneath the word "send:". Ask him to copy
these numbers (the easiest way is to highlight them and press CTRL+C to
copy) and paste them into an IM message to you.
When you receive the IM, copy the numbers from it and paste them into
the "receive" blank on your screen (you can press CTRL+V to paste), and
then hit the "GO" button. The game will play out your opponent's turn,
after which you take your turn. At the end of your turn, you will see a
set of numbers to send to your opponent.
Follow the same instructions above that your opponent used to send the
numbers to you to send the new numbers back to your opponent.
Continue IMing moves back and forth until one player wins.