Backlight keyboard commands working on live CD but not on installed ubuntu

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http://askubuntu.com – so basically when i put the ubuntu installation disc of Ubuntu 11.10 inside and run a Live session everything works fine (i can press Fn + Arrow Up and Fn + Arrow Down to control the screen Backlight). But after that i install the system, it doesnt work anymore... but other commands (like Fn + Arrow Left for lowering the volume) are working i've installed latest nvidia propretary drivers (and maybe thats the issue?), coz i dunno what other drivers i could use waiting an answer (HowTos)