Are you going through the Chromecast app to begin with? You have to start the process through them, not Cathe Live.
Go to Chromecast. Use their search window to type in a Cathe Live. I usually type Cathe Streaming, and one pops up. Click on that. It will bring you to the Cathe Live page. Click on "my account". Then on "my on demand videos". Once you find which video you want, click on it. Then click the video to play. It takes a few seconds, but then at the bottom of your phone screen you will see the window to cast fill with a bunch of words. Click that, and it should cast to your TV.