What are you trying to do specifically? Just get it working in the first place? This confused me at first too.
Have you downloaded the files from the PinSound forum? You have to load the files onto the USB drive after downloading, then insert into the PinSound board, then turn the game on. The game will do what it needs to do, assuming you've loaded them correctly.
It doesn't work anything like Pinball Browser, where you just swap through - the files are not accessible just from your game unless you're set up to record them (you're not, and there's no reason to do so
Once you've loaded several mixes onthe USB, you can switch between them by reducing the game volume on the coin door to 0, then bringing it back up loud