The esc key will bring up a dialog asking you to confirm that you want to quit the present game. pop-pop will be paused until you dismiss this dialog. To resume a game after you have paused it, press the esc key once more, or simply click the Cancel button.
The Chat function will also pause a networked game. Press the return key, and the game will be paused until you click on the Cancel button.
To quit the program quickly, press command q. This will exit pop-pop and return you to the Finder.