Go to Account > Hosting Plans
Click Create Hosting Plan button
Edit Hosting Plan page will appear. Give this hosting plan a name and select previously created Virtual Server as a Target Server.
Wait couple of seconds and you will see that System, Service Levels and Web Sites quotas are available for selection now.
We need only 2 of them: System and Web Hosting.
Setup limits according best practices and your wishes. Click Save button at the bottom of the page.