News+ on Mac
I signed up for News+ on my iPad - Everything works fine. When I try to access on my Mac it asks me to sign up - Do not see an option to log in to the existing account
MacBook Pro 13”, macOS 10.14
Well, assuming you have News enabled in System Preferences > iCloud and you are using the same Apple ID on all the devices it should work. At least it worked for me with the 30 day free trial.
You can wait for more answers or contact Apple Support.