You can go to playback page or use VioStor Player to convert multiple recording files to AVI file.
After converted, you can play the recording file of QNAP NVR on Mac OS.
About the tutorial, please visit http://www.qnapsecurity.com/App_Video03.asp.
Please check the video from 3:24 to 3:47.
And you can play the converted video on Mac OS.