@Wantuil > First off, I recommend reading the MIDI section of the Serato manual.
You need to open the Setup screen inside Scratch Live and click MIDI. You need to ensure the CDJ900 is showing up here and also ensure you have enabled MIDI devices.
Now click MIDI in the main Scratch LIve screen and start MIDI mapping. Serato's guide shows you how to MIDI in Scratch Live : serato.com