Try this link... It's explane very good.
http://pioneerdj.com/support/product.php?c=2501&lang=en&p=DJM-850-K&t=121
Don't forget the aggregator-app
There is too ways you can conect the system.
"Soundcard" useed bye DJM 850. Our from the CDJ 900.
If you use the cdj 900 for soundcard conecting, Than you need the aggregator-app.
The aggregator-app is for Mac Only.
If you use the DJM 850, don't forget to download the drivers for the mixer DJM 850.
Theres a big difrence and dipence what way you like to work.
I preferm you try the DJM 850 conecting first for soundcard.
Anyway...! You need an USB Hub.
Good luck!!!! And have fun!