TweetCaster uses an application called "Boxcar" that updates you of your notifications through Twitter's API. You can download the application from the following link:
When you go into BoxCar > click "add a service" > click on "twitter account" > in the advanced settings area (located towards middle of screen), please click on "Opens:Boxcar" and it will direct you to the "choose your client" screen. From this screen, please select "TweetCaster", which will be located towards the top in the "Clients you have installed" area. If you have any further questions or issues, please let us know.