If you've ordered a Windows VPS, you will need to set an admin password over VNC to activate RDP functionality.
To do this, please navigate to your service(s) page under My Services in your client area. Here you will see a brief control panel with basic controls/actions, including VNC - see below.
Once the popup for VNC has opened, please select HTML VNC.
After a few seconds, you will see your VPS/Windows screen come up for you to follow the on-screen instructions and set a password.