In your case (for now) leave the www part empty.
When asked for the 'enter the domain' part, use the server IP (if on a LAN or behind a router use the local server IP)
You can change all this later when you login on ISPconfig.
To access ISPconfig (after using the IP setup), use http://IP:81
(or if you told ISPconfig to use https, use https://IP:81
An other option (if you did install it with the domain name) is to add the domain name to the hosts file of the PC that you are trying to access ISPconfig with.
When adding the domain name in the HOSTS file, make sure that you set the correct IP of the server.
The hosts file for a Windows PC can be found in c:\windows\system32\drivers\etc