Hello I am using the latest Mac Book Pro and the Audacity version 2.3.0. I have two CDJ’s and a Pioneer Mixer DJM-250MK2. I have my laptop connected to my mixer via USB. To my knowledge my mixer has a built in audio interface. I have input and output set to my mixer but Audacity will not record. All I get is a flat line and no play back. I have tried changing the settings many different ways but still nothing. The only way I can record is if I change the settings in the tool bar to “built in microphone” and “built in output” it’s as if audacity can’t pick up sound from my mixer. I can’t not record using the built in mic because of the hollow sound. I am new to all of this so sorry if I’m having a very difficult time. Please help I have to submit a recording soon!! 
To my knowledge my mixer has a built in audio interface.
You still need to plug in some kind of microphone to your mixer. It does not have the capability of recording your voice. You need a signal source (a microphone, a turntable, etc.) that you plug into it, whose output (your voice, a record) you will be recording.
Maybe you are trying to record the sound from your CDJs that you have plugged into the mixer. If that’s the case, ignore that comment.
I do notice that the Amazon product page for your mixer has Q&A’s and reviews stating that “The sound card built in is exclusively for Pioneer’s Rekordbox DVS software”. That may be the issue.
It suggests strongly that you should be able to do what you did. Pug it into your computer and record the work.
Can you hear the music at the mixer headphone connection?
Apple (upper left) > System Preferences > Sound > Input. Can you see the mixer there and can you make the sound meter bounce?
Koz
Actually I had this very same issue with our new Mackie 324Vlz USB record. Here is what I found I had to do that flat line is not really a dead signal at all, run about a 30 second recording using your vocal channel on your board. Have Audacity record this. Now you have your 30 second test recording do this once you click stop to stop the recording go to effects and click on normalize. you will want to make sure to remove D/C offset is selected and set your normalize audio amplitude to -10Db then click ok it will now normalize the 30 second audio you just recorded if you click play you will now hear what you just recorded. you can also click on the downward triangle to the left of your recorded track and select Db waveform and that will change that flat line to an actual waveform that you can see. I know this is kinda long but it is what you have to do. Also one final note under preferences you will also want to change your audio sampling to 24 bit instead of 16 bit as your mixer is sending out 24 bit audio this is found under the quality setting in preferences. change the 16 bit to 24 bit and save. good Luck.
I tried this and I can actually see the wave form now but there is still no sound when I push play to hear the test recording.