Comentario oficial
I know sometimes the window can have scrollbars with the devices listed off the visible area, can you please check that it's not sitting elsewhere on the screen?
If it's not visible, and is nowhere to be found, follow these instructions:
1) Create a new configuration from the drop-down (yours probably says "Default" as mine shows):
2) Name the new configuration:
...and click OK. The currently connected MIDI devices will be scanned; any device not connected will NOT appear on the screen (such as the DDJ-RB, which you can see "ghosted" in the image above).
3) If your XDJ-AERO does not appear, click the re-scan button:
4) If it STILL doesn't appear after that, click the Apple icon (top left), then hold the Option key on your keyboard and click System Information...
Click the USB item on the left tree, and if your DDJ-AERO appears on the list at the right, click it:
Post a screen-shot of that window (command+shift+4, then hover the mouse over that window, press the space bar, then click - the screen shot will be saved to your desktop).
