See this:
http://docs.whmcs.com/Servers
Quote:
Adding a New Server
We have prepared a video tutorial for this section @
http://www.whmcs.com/tutorials.php?i=5
You can add new servers by going to Setup > Servers and selecting Add New Server. All fields are optional but you should at least enter a name and IP for each server you add:
Name - should be unique to allow you to identify this server
Hostname - this is the primary domain of the server - used to link to the server by certain modules
IP Address - this is the primary IP address of the server - used to connect to it
Monthly Cost - optional field - used in reports to calculate profit if entered
Datacenter/NOC - optional field - used only for you to remember where the server is located
Maximum No. of Accounts - a value used to work out the percentage usage of the server (must be entered, can be very large)
Primary/Secondary/Tertiary/Quaternary Nameservers - these are the nameservers for this server, eg. ns1.yourdomain.com and ns2.yourdomain.com - used in welcome email and when registering domains
Primary/Secondary/Tertiary/Quaternary Nameserver IPs - these are the ips for each of the nameservers, can be used in emails
Server Type, Username, Access Hash and Password - should be the login (root/reseller) details for your server (Or Username/Access Hash for cPanel/WHM Servers)
Server Status URL - This is the url to the status folder. See Status Monitoring below.