You can login into the root user by first logging into the ploi user and then typing
sudo su, this will prompt you for the sudo password which you received in the e-mail on server completion. By default root login is disabled so you always have to login through the ploi user because of safety precautions.
When you get the password prompt, bear in mind that you will not 'see' yourself typing the password, this is hidden. When done right, your terminal looks something like this: