You must create DNS records on the authoritative name servers for your domain. These can be your registrar's name servers or your ISPConfig servers or any other server. You can find these out by running
dig ns yourdomain.com
If your ISPConfig name servers are authoritative, you should take a look at this guide: http://www.howtoforge.com/how-to-run...debian-squeeze
For testing purposes (if you don't want to change DNS records), you can modify your client PC's hosts file.