When using Chromecast-enabled devices, you shouldn't need to "add" a device or do any setup. If the device you are casting from (such as a laptop, your phone, or a tablet) is connected to the same WiFi network as your TV, they should automatically see one another. One thing to keep in mind is that some content does not allow casting or certain apps or websites may block the ability to cast. You also choose to cast from the device providing the content, not on your TV.
Does that clear things up for you a bit? If you're still having questions or something still isn't working right, let me know.