A:
Be sure that you are using Google Chrome as your browser and download the Google Cast Extension here:
https://chrome.google.com/webstore/detail/google-cast/boadgeojelhgndaghljhdicfkmllpafd?hl=en
If you're 100% sure that it's set up (try casting from your phone or tablet), then reset the device and try again. If that does not work, ensure that you're connected to the same wireless network (same SSID) as the Chromecast.
As a last resort, try setting it up again:
https://support.google.com/chromecast/answer/2998456?hl=en