Welcome to Seven Forums vadlamudibi.
For adjusting brightness just press F5 & F6 key's without Pressing Fun key.
If it still not work right click on the battery icon in lower right and choose Power Options
A window will popup there you can see your laptops power planes
Like 'Balanced' 'Power Saver' etc(it is good to put your laptop on Balanced).
Click on Change plane settings
. There You can adjust your laptops Brightness
Eather when pc is on battery or on AC. Adjust them As you like And that's it.
Edit: Hear you can Read a detailed tutorial on this: Power Plan Settings
And hear you can read many more grate tutorials on Power Management