1. Download the latest version of the "Boot Camp Support Software" (e.g., 5.1.5722) from https://support.apple.com/downloads/bootcamp
2. Unzip the file (e.g., bootcamp5.1.5722.zip) to a new directory
3. Run BootCamp\Drivers\Apple\AppleWirelessMouse64.exe
4. Turn on Bluetooth and pair your Magic Mouse

Note: Apple wireless mouse devices have a built-in passcode of "0000" (four zeros).