Plug the source via usb on the Mac and turn it on
Open the Audio and MIDI Setup on the Mac : you should be able to see the usb source on the left side and choose the sample rate.
Only then, open Audacity and select Devialet USB 2.0 as input
You should be able to get a signal and to record your files from there.
Please find attached more information about how to use the USB bi-direction: link