1) Set up all the hardware connections (computer USB to 'USB DAC' on the 602, and 602's Line Out to your amp) and switch everything on.
2) Go to System Preferences on your Mac, select 'Sound', and choose the 'Output' tab. You should see your 602 as one of the output devices. Select this instead of 'Headphones'.
3) Now go to your Applications folder, choose 'Utilities', then 'Audio MIDI Setup'. Again you should see the 602 in the list of devices on the...