I do not understand how to login. When it says "Unix user’s username and password" I am not sure what it is referring to. On the Linode instance I am root. I have tried entering 'root' as a username and my root password.
This link refers to users on the Linode cloud service account, these are users which can create Linode instances. I am unclear if they can be automatically added as users on a specific Linode instance.
Nevertheless, once there I created a new user on the instance using adduser and that allows me to login to the Linode instance as a normal user. It also provides the username and password I need to login to the rstudio-server running on the Linode instance.