Other symptoms: LabQuest Mini has power, but the LED stays red. LabQuest is plugged in, but the screen hasn’t changed to the double arrows “connected” screen.
This can happen when you have VMWare Fusion running, because by default it asks you whether you want it to connect to Windows or Mac when you plug it in. If you don’t dismiss that window, the device is in a limbo state of receiving power, but not communicating with the computer.
Close VMWare. Plug in LabQuest interface. If it works as expected, run VMWare. When it asks which system to connect to, be sure to select: Always connect to this machine (approximate wording).