To route playback audio from your computer to the X32 you must first make sure the audio is routed correctly in your DAW.  If you want each track in your DAW to show up on individual channels on the X32 you'll have to set each output to the corresponding track.  
Once your DAW is configured, you will need to configure the X32.  
1.) Press the ROUTING button2.) Go to the HOME tab (on the left side of the screen)
3.) Set Inputs 1-8 from Local 1-8 to Card 1-8
4.) Set Inputs 9-16 from Local 9-16 to Card 9-16
5.) Set Inputs 17-24 from Local 17-24 to Card 17-24
6.) Set Inputs 25-32 from Local 25-32 to Card 25-32