To use twittercast you need to have twitter set up on your device. This is done is Apple's Setting app.
From the "Collect Data" page, the menu in the upper right corner gives you access to the Twittercast settings. From there you can enable the auto broadcast and the message content and frequency.
When Twittercast is enabled, the Twitter icon (a blue bird) will appear in the upper right corner of the collect data page. That icon also serves as a menu when visible which allows you to send out the current score and edit that message. This is useful to signal the end of the game since the app does not know when the game is over so does not autosend such a message.