the guide on godaddy is obselete you have to
-open the domain manager
-click on the domain name you want to have nameserver extenstions
-scroll down to hosts and select add hosts.
-add ns1 then IP address
-add ns2 then add the different IP address
This sets up your glue records for ns1.mydomain.com
If you dont do this it will not let you point your other domains to your domain as a nameserver.
Make sure you use ns2 on a second box with ISPconfig configured as a slave as it tells you to in the instructions Falko posted the links too for your second nameserver to work right.