First, make sure that you are casting from a mobile device (a tablet or a mobile phone) or from your laptop's Chrome browser. Second, make sure that both devices are connected to the same internet connection.

Casting from mobile web browser on your mobile phone or tablet isn't possible)

If you are casting from a Chrome browser on your computer and you cannot see the Chromecast icon, these instructions should help.

For more troubleshooting tips, please refer to Chromecast's support page here.