Hi Lukas,
Yep plug the USB into the top of the mixer and then into you're laptop. Make sure you have the DJM 900 utility installed (you can download it from Pioneer), this will assist with sorting out recording levels etc. Load up Audacity, select DJM 900 and then simply hit record and then you are good to go mate. Very simple, very easy to set up and the quality is pretty good.
Hope this helps mate,
Kenfold