That's because com.google.cast
is not part of Android.
Near as I can tell, you will need to download a ZIP file containing their library project, import it into your IDE, mark it as actually being a library project, then attach that library project to your app.
Actually using those APIs appears to require a whitelisting, and it is in violation of their license to actually distribute an app (or, near as I can tell, source code) without express written permission of Google.