@Eric > Connect your CDJs via USB to your MacBook. Ensure they are powered on.
Download the MIDI mapper and CDJ Aggregator from here: http://pioneerdj.com/support/product.php?lang=en&p=CDJ-850&t=93
Unzip the CDJ Aggregator on your computer. Open and run it - you should see both your CDJs within the window - Click OK.
Open up Traktor (pretty sure there's a setup wizard for the 850). Switch to PC mode on your CDJ.
Open the audio preferences within Traktor and select PIONEER CDJ
Go to the Output Routing options and choose Deck A 0;1 and for Deck B 2;3
Then go to the Controller Manager and choose ADD then IMPORT and select the MIDI Mapping that you downloaded from the above link.
After that, you should be ready to play. Please let us know.