Not sure if you're playing back from the computer or just using it stand alone.
If you're recording to the computer and want to hear audio back from the computer into your monitors / headphones.
Make sure you have the USB Return button engaged on the super channel so that you can monitor audio back from the computer.
That fader is label ST 17/18 that is your Output 1/2 from your computer, it's the same channel with the Bluetooth button on it.
To the left of that is 15/16, just below the Mic Trim knob there is a button 3/4 with a USB symbol, that allows outputs 3 and 4 from the computer to be monitored through that channel.
Both of these feed the monitor bus, use the main fader to control the overall volume to your speakers
plug in headphones and use the phones knob below the main VU meter to control your headphone volume.