The 'NOW TV Box' is a highly subsidised (cheaper) version of the 'Roku Box', which is one of reasons why it lacks many of the features and apps that the Roku products do!
You can't have your cake and eat it. If you require more features and apps, you'll need to buy the far more expensive Roku products 😉
Just to back up what SMD has posted, the Now TV boxes are structured/designed to mainly deliver Sky and Now TV content, hence the highly subsidised price. In other words, your gateway into "Pay TV" without being burdened by a fixed (and expensive) contract.
However, there is some minimal flexibility built into the Now TV boxes that will allow you to add only one test channel/app, and most people choose to add Plex as their one and only app.
I, myself, use Plex to play my old albums/CDs (MP3s bought and downloaded from Amazon) via my NAS media server to my TV based sound system, all made possible by my Now TV box.
You know that you are asking me a question based on a posting I made nearly 3 years ago and in that time a lot of things, in this fast moving tech environment, has changed.
So much so that Now TV has since blocked the ability to add anything, i.e. channels, facilities, etc., that is not in their Sky/Now TV App/Channel Store. Therefore, if AccuRadio is not in Now TV's Channel Store you cannot add it to your Now TV device.
And BTW, that's why I and many others have ditched our Now TV devices and now own Roku devices instead.