Yeah that was my problem. Now I have a new problem.
It installed the extension, but each time I tried to play a YouTube video it wouldn't. I rebooted the Chromecast. It downloaded an update, and now Chrome doesn't see the Chromecast despite the fact that it works perfect with my cell phone on the same network.
I have restarted Chrome several times (and even ended task on it afterwards) but it still doesn't see it. I also tried disabling the Windows firewall, but no joy.
Lastly I tried uninstalling and reinstalling the Chrome app. No help.