First Double click the network icon in the Control Panel:
Then select the TCP/IP protocol for the local network card and click the "Properties button":
Under "TCP/IP Properties", select the "DNS Configuration" tab, check "Enable DNS", and enter the IP address of the local DNS server (*) in the "DNS Server Search Order" box, and click the "Add" button:
Finally click "OK" both in the "TCP/IP Properties" and "Network" dialogs to save your changes.
(*) The DNS server IP address must match an IP address that Simple DNS Plus is configured to listen on in the Options dialog / DNS / Inbound Requests section.
If you are configuring the computer which Simple DNS Plus is running on, you can use 127.0.0.1 (the "localhost" address) - otherwise you must use an IP address which is accessible over the local area network.