Login to your Control Panel, Search for the domain name for which you have purchased this Order and go to the Order Information view. See details
Click the Name Server Details link.
Here, you would find the list of Name Servers you need to set for your domain name. See details
Alternatively, if you wish to retain the Name Servers of your domain name instead of using BigRock's Name Servers, you would need to add the DNS Records listed on this page, on your existing Name Servers.