To do so:
1) Login (Duh! )
2) Click 'manage domains'
3) Click on the domain you want to add it to
4) Click the + next to the "Domain Host Summary"
5) Click "Click here to see details or to modify."
6) Add them in, one at a time, associating the ns1 with an IP address and the NS2 with an IP address
7) Wait. They take time to 'propegate out'.