Make sure that the universe repository is enabled.
Execute the following commands in a terminal
sudo apt-get install numlockx
sudo gedit /etc/X11/gdm/Init/Default
Add the following lines at the end before the line "exit 0":
if [ -x /usr/bin/X11/numlockx ]; then
Put off your numlock and restart X by pressing Ctrl+Alt+Backspace
The numlock should be activated now and every time when you reboot