Right-click the Windows Start button, and select "Network Connections":
Then in the "Settings" window, click your primary network connection:
Scoll down to "DNS server assignment" and click the "Edit" button:
In the "Edit DNS settings dialog", select "Manual" in the first drop-down, enable the "IPv4" switch, and enter the IP address of the local DNS server (*) as the Preferred DNS server:
Finally, click the "Save" button in this dialog 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.