Copy the Pioneer-DDJ-REV1-script.js and Pioneer DDJ-REV1.midi.xml files into your User Mapping folder.
%LOCALAPPDATA%\Mixxx
Mixxx 2.3: ~/Library/Containers/org.mixxx.mixxx/Data/Library/Application Support/Mixxx
Mixxx 2.2 and earlier: ~/Library/Application Support/Mixxx
~/.mixxx/
This resolves to /home/<your-username>/.mixxx
Restart Mixxx and load the mapping.
For more infomration follow the official Mixxx guide.