Sounds like you need to pair your new remote with your chromecast. Here's the google help article for that.
If you're just adding a second remote to your chromecast, you can set that up in settings under remote.
RT63, thanks for the information.
Are you able to pair your Voice Remote to your Chromecast with Google TV using the steps above? If not, try resetting your Chromecast so that it will trigger the pairing mode of your new remote.
Here are the steps in resetting your Chromecast:
Let us know how it goes.