I don't know about getting the keys to work but if your laptop uses an nivida card you should be able to dim the screen through the nvidia settings package available through synaptic. This of course depends on you having an nvidia card, do you know what is doing the graphics? is it a graphics card or onboard graphics?
This might be worth a look if your laptop is hp: http://ubuntuforums.org/showthread.php?p=4178093#post4178093 or i have also seen mentions of a brightness app, im guessing available through synaptic (im on xp so i cant check). Seems like this is common problem specifically for ubuntu though.