When you execute
what number is returned?
What distribution do you use? Perhaps you don't have sudo installed, so you need this command instead
su -c "echo 50 | tee -a /sys/class/backlight/nvidia_backlight/brightness"
You can also test the brightness with the following command. Run it as normal user, not root.
dbus-send --print-reply --system --dest=org.freedesktop.Hal /org/freedesktop/Hal/devices/computer_backlight org.freedesktop.Hal.Device.LaptopPanel.SetBrightness int32:50
P.S. You can try lower values than 50