The way I achieved it was by configuring Fldigi to capture its data using the built-in microphone and turn the volume in Web SDR up until it was sufficient. The result was surprisingly good; of course partly because the Mac has relatively good sound hardware – at least when compared to some standard PC sound chip.
I prepared this video demonstrating how it works. You will hear and see many PSK-31 stations as well as some RTTY and other modes. Enjoy and have fun.
Watch in high resolution.