To add a server and configure it with a plugin like cPanel, navigate to Settings - Products - Servers and you can choose to add a server. You will need to enter the information required, and select the type of server plugin from the drop down.
For cPanel, you will need to get the access hash from WHM and enter it in along with your username.
You can also choose to enter a Status Message, that will appear to your customers who are on that server.
You will then need to view your hosting product and select Advanced & Plugin Settings and enter the relevant information and choose the servers that it applies to.