Discovery Plus isn't actually a channel. per se. It's this collection of channels:
https://www.discoveryplus.co.uk/channels
You can sign up for free to gain access to a small selection of channels from within that collection, plus catch-up streaming from those same channels:
https://www.discoveryplus.co.uk/free
To get everything though, you have to pay either £4.99 per month or £49.99 per year:
https://www.discoveryplus.co.uk/myaccount/products
There's no app for the service on the vast majority of Smart TVs at the current time though, so you either need one of the latest Samsung TV sets or an Amazon Fire TV stick (if you're not getting the service through a Sky Q box):
https://www.discoveryplus.co.uk/how-to-watch