1) With a CDJ or other line level device connected to a channel, adjust the gain so that channel meter shows activity around the first orange LED. Take the channel fader to full-open.
2) If the crossfader is assigned to that channel, ensure the crossfader is set to that side.
3) Turn the master volume knob to about 2 o'clock - the master channel meter should show activity.
4) Check your output on the master RCAs to see if whatever you're connected to is getting signal.