There are 2 places for DNS on that device as there are 2 products. The Fiery is on your network and needs to have DNS set, which is done via Command Workstation, login as admin and go to Server > Configure > Network > Protocol > TCP/IP > DNS then Apply > Reboot
Then there is the J75 itself, go to Webtools and select the Xerox Color J75 Press CentreWare link at the lower right
Select the Properties > Connectivity > Protocols >TCP/IP section and check the DNS at the bottom, the preferred DNS server should be 100.100.100.102, the other 2 can be whatever you use typically. It is also highly suggested something be in the domain name field, even if just MSHOME or Workgroup