OK. I have seen your previous message now and can see that you are probably referring to Bigpond Movies.
Good news, Chromecast functionality was introduced almost 2 months ago. What you may not have caught up on is that the app is now called Telstra TV Box Office. So you will need to install that and you should be up and running.
Note: a Chromecast device counts as one of your registered devices (so tablet + chromecast = 2 registered devices out of the 5 total that you can have).
Never be afraid to back yourself when trying new things, just always make sure you have 3 escape routes if things go wrong.