Controlling OmapL138 EVM backlight

I am using the OmapL138 evaluation board and need to control the LCD backlight (userspace slider)

I built the 03.22 kernel using instructions here http://processors.wiki.ti.com/index.php/LCD_brightness_control_on_da850/omap-l138

But there is no brightness change when I set the brightness value like so.

echo 10 >  /sys/class/backlight/pwm-backlight/brightness

What would I need to do to get the LCD brightness to work?

Thanks in advance