can´t adjust bright

[Manu <manu@sindominio.net>]

I have Debian unstable on a Powerbook G4 12", and have installed
pbbuttons (apt) and it works, because i can change the volume. But i
can´t change the screen bright. 

In my /etc/pbbuttons.conf, i have:

#Brightness             = 7      ; initial brightness level
#BrightnessupKey                = 225
#BrightnessdownKey      = 224

And when i execute showkey an press f1 and f2, i get the message:

keyboard: unknown scancode e0 4c
keyboard: unknown scancode e0 4c
keyboard: unknown scancode e0 54
keyboard: unknown scancode e0 54

Could you give me a hint?