If a USB device isn’t showing up on your Mac, your first action should be to confirm the problem isn’t with the charger.  Safely disconnect your iPod and try connecting to another device. If the issue persists, the problem may be with the cable. Disconnect it and try reconnecting with a different USB port.  If nothing works and your USB device has a Bluetooth option, one way around the problem is to give up on USB altogether and try to get your Mac to recognize an iPod or iPhone via Bluetooth connection.