Acer Aspire 5610

If u check your keyboard you'll notice there's a button marked 'fn' , might be in a different color,now if you look good you'll notice a little star object usually on the left and right arrow key,now to use this additional function u need to press and hold the 'fn' button then to adjust the brightness press the right arrow key until seem fit or the left to adjust.

Look under the power saving settings in control panel