@Lud > You can record from the Nexus using the free Audacity software thats available for PC and Mac. You'll need to connect the mixer to your MacBook via a USB cable and then configure the settings.
You can watch Pulse configure the settings here: (note he does it on a DJM2000 but the steps are the same for the 900) http://www.youtube.com/watch?v=Hnq7M6g4hlA&feature=youtu.be
You can find the necessary downloads here: http://pioneerdj.com/support/product.php?lang=en&p=DJM-900NXS&t=111
Please try it out and if you have any problems, get back to us and we'll endeavour to help you.