Hello,
A domain registrar will require you to use 2 IP address because the normal way of doing it is to have 2 dns servers. DA simply gives you the tools to add 2 IPs to one machine so that you can fool your registrar into thinking you have 2 servers when you actually only have 1. (2 servers are more reliable, but 1 is adequate for todays systems).
To add a 2nd IP, you'd go to the Admin Panel -> IP Manager and add the IP to the system. DA handles the adding of the IPs. You should be assigned the IP's from your datacenter.
Once added to the system, then you can assign the IP's to the Reseller who you want to sue to create your nameservers (IP Manager again in Admin Panel). Then go to: Reseller Panel -> Nameservers, select the 2 IP's, then domain and create the nameservers.
John