So, to turn them on:
sudo gpioctl -f /dev/gpioc1 21 1 sudo gpioctl -f /dev/gpioc1 22 1 sudo gpioctl -f /dev/gpioc1 23 1 sudo gpioctl -f /dev/gpioc1 24 1
Or, access them via the led driver under /dev/led
% ls -lA /dev/led total 0 crw------- 1 root wheel 0x33 Feb 3 15:58 beaglebone:green:heartbeat crw------- 1 root wheel 0x34 Feb 3 15:58 beaglebone:green:mmc0 crw------- 1 root wheel 0x35 Feb 3 23:44 beaglebone:green:usr2 crw------- 1 root wheel 0x36 Feb 3 15:58 beaglebone:green:usr3