Not much experience with recording to an iDevice but I do record mixes directly into Logic. The process should be similar or identical in Garageband.
Connect your DJM-2000 through USB to the Computer with the recording software
Go to your audio Settings and make sure your DJM is set to 6 output & 2 Inputs
Open Garageband
Go to Preferences--> Audio
Make your DJM-2000 the "Input device"
Create an audio track, make sure DJM-2000 is the input device for the track
You should be good to go!
Like I said, not sure if garageband will be identical but should be close. The audio quality of the recording will be better using this method. The cables on amazon can be sketchy sometimes. Hope this helps!