You could use the -audiodev flag (
http://en.flossmanuals.net/PureData/AdvancedConfig) but I never used it on Mac, so I'm not sure whether it is appliable; with flags you could also specify sound servers/API to playback with. So another solution could just be to use jack (I usually reccommend to install jack on Mac for better flexibility and efficiency). Then you would start jack with an appropriate profile and Pd with -jack and -channels flag and you're done.