Thanks for the help. I was able to do it. Not sure if this was really what made the critical difference but if you're interested, I shut down all mac machines except for the Macbook Pro. The Chromecast dongle i was trying to reach was the newest version, 2.0.
What's cool is that my Samsung TV is about 7 years old, my Macbook Pro is from late 2011, and my desktop iMac is from 2012. But I can cast successfully to all now. The advantage is that I can now watch Amazon Videos, using Google Chrome as my browser, and watch it on my TV which has a big screen. This enables me to stop having to move a heavy desktop machine, and to avoid buying a smart TV for hopefully another year.
Thanks again.