Function keys doent work by default in kde 4.2. Those who migrate from gnome to kde 4 find it difficult to adjust without the 'fn' keys. The problem is with powerdevil. The new applet does not have options to configure the acpi events. This will exits till powerdevil is fixed. As of now, we can use a python script to handle these events. Make sure python-2.6, python-dbus and python-xlib are installed in your system. Download the script from here or here.
Give exe permission
#chmod +x softkeys.py
Place the script in ~/.kde/Autostart/ and restart your system. The function keys must start working now.