The installation procedure is the same as for the old NanoStation 5 but you have to use this image:

http://downloads.openwrt.org/backfire/10.03.1/ar71xx/openwrt-ar71xx-ubnt-nano-m-squashfs-factory.bin

Power cycle the device while you keep the reset button pushed. Leds will start to blink red and yellow.

Then tftp the image to 192.168.1.20 following the instructions at the Installing OpenWrt via TFTP page.