If you are using the proprietary drivers, you can install a program called
nvclock (to do this you can search nvclock in the Ubuntu Software Centre).
You can then go to System -> Preferences -> Keyboard Shortcuts and create 2 new Actions (brightness-up and brightness-down). Set the commands to
nvclock -S +5 and
nvclock -S -5 respectively. Next choose a shortcut for each command. Normally, you can use your brightness fn keys to do this (eg. on my laptop fn+F7 and fn+F8).
You can then use these shortcuts to change the brightness.
At any time you can press alt-f2 and enter:
nvclock -S x where x is a number that is the percentage screen brightness; eg.
nvclock -S 100 sets maximum brightness. You can bind these to shortcuts too and use a terminal instead of Alt-F2 if you wish.
You can find out more about how to use the nvclock command by entering
man nvclock in a terminal.