Official comment
I would say if you don't have at least 2.02, you should update:
Ver.2.02 (8 August, 2017 Update)
[IMPROVED]
- Cut lag length improved for a more responsive scratch performance.
[FIXED]
- Popping noise occurred via USB audio (FX SEND/RETURN) when switching FX assign on rekordbox with BEAT FX enabled.
- Fixed other minor issues.
If it's not recognized as an audio device, is it recognized as a USB device? Go to the Apple icon, click that and select About this Mac... and then click the System Report... button. On the left, click USB and then look on the device tree at the top to see if the DJM-900NXS2 appears while connected. If not, then try a different USB port / cable.
If the DJM does appear, but still isn't an available audio device, it could be you didn't bypass the system security when installing the drivers (High Sierra / Mojave). First, download the drivers, run the uninstaller from inside that pack, restart, then install the drivers, and follow these instructions to allow the driver install through the OS security.