Your root login password should be in the welcome email you receive when you sign up for VPS services. To confirm this password, go to the client area, and click the "My Services" tab;
You then need to expand the product you wish to view the password of;
This will bring you to the relevant screen where you need to scroll down past the stats graphs of your virtual machine. There should be 3, day, month and year. If you have just started your virtual machine, bear in mind these graphs won't be displayed for at least 10 minutes. You should then see your login details;
The login you need to use on the server is root. The password is displayed after the VM name, and should be the current one you need to use.