Scratch has long had a "sensors" block which allowed users to connect to a PicoBoard, but in version 1.3 a feature was added to let any application talk to Scratch. To enable this feature, a user must enter the Sensing Blocks palette, right click the sensors block and choose "enable remote sensor connections". The application will then open itself up to communication on port 42001. If you're using Vista or a similar dastardly operating system you may have to confirm this action through a system dialog.