Solution using halevt
According to the man pages halevt is a generic handler for HAL events. It is deprecated and replaced by udev, but since I don't know enough about udev I will give a halevt solution.
EDIT: after some serious sweat, I managed to do this in udev. See my other answer.
I will use vim to edit files, but if you don't know vim you can replace it by nano or gedit.
Installing halevt
sudo apt-get update && sudo apt-get install halevt
Determining which events you want to bind to a script
Stop the halevt daemon which is already running:
sudo /etc/init.d/halevt stop
Now see if halevt can recognize the events of the keys you want to use, start the listener:
sudo -u halevt halevt -fig:plugdev
Now press the function key on your keyboard you want to bind the script to. I know that the OP wants to get his brightness keys working, so let's go with that. The output for the brightness keys should look something like this:
Condition: /org/freedesktop/Hal/devices/platform_i8042_i8042_KBD_port_logicaldev_input,ButtonPressed (brightness-down)
Condition: /org/freedesktop/Hal/devices/platform_i8042_i8042_KBD_port_logicaldev_input,ButtonPressed (brightness-up)
You can see that the brightness-down and brightness-up events are transmitted.
Bind the event to a script
Now edit the file /etc/halevt/halevt.xml:
sudo vim /etc/halevt/halevt.xml
and add the following lines ( I did it at the bottom, just before </halevt:Configuration>):
<halevt:Device match="hal.info.category = input">
<halevt:Condition name="ButtonPressed" value="brightness-up" exec="sudo /home/user/brightness-script.sh up"/>
<halevt:Condition name="ButtonPressed" value="brightness-down" exec="sudo /home/user/brightness-script.sh down"/>
</halevt:Device>
where of course you should change value to the event that you got from the listener, and exec by the command you want to execute.
Give the halevt user permission to do the command or script
Since the halevt daemon is run as the halevt user you have to give it permission to do what you specified in exec.
Run (remember to replace vim by your editor of choice)
sudo EDITOR=vim visudo
and add the following lines at the bottom
halevt ALL=(root) NOPASSWD: /home/user/brightness-script.sh
and save and quit.
Make sure your script is executable
sudo chmod +x /home/user/brightness-script.sh
Start the halevt daemon again
sudo /etc/init.d/halevt start
And it should be working!
Best Answer
showkey --scancodesworks if you run it from a console window, which you can open via Ctrl+Alt+F1. (Use Ctrl+Alt+F7 to close it afterwards.)