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.
Following the Getting Started with Linode it appears you have to initially log into the instance as root.
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.
Thank-you for your help