What Chromecast generation model are you using?
Where you download the NOW App to your smart phone or tablet rather than Chromecast itself.
Or do you mean you have Google Chromecast TV with the remote where the native NOW App is not supported from the Google Play Store.
Unfortunately at the moment there is no native NOW App support by NOW themselves to use on the Google Chromecast TV.
So you will have to use for now casting by adding the NOW App to your smart phone or tablet.
Maybe send NOW a tweet or drop them an email to see if an official native NOW App support is in the pipeline for Google Chromecast TV.
Google Chromecast TV with the supplied remote is a different Beast running Google / Android TV compared to the Chromecast dongles listed on the NOW "ways to watch" link and shouldn't be confused with the Chromecast Dongles listed.
Until NOW decides to rollout more support to Google TV or Android TV devices then you will have to look at other workarounds such has casting from your mobile device to the Google Chromecast TV device.
Perhaps send NOW an email asking if there is any inbuilt NOW App support for these such devices on the horizon.
Here's their email address on the screen shot below taken from their NOW Twitter feed.
Some good news for Chromecast Google TV owners who are after the native NOW App on it.
Have a read of this internet link article below.
The NOW App was rolled out on my Chromecast Google TV earlier today 😁.