If you've received or won a Promo Code, you can use it to get the indicated app for free.
To do that:
1) Go to App Store app on your iPad or iPhone
2) Make sure you're in the 'Featured' section (bottom menu)
3) Scroll all the way down
4) Tap the 'Redeem' button at the bottom of the page
5) If asked to login, enter your password
6) Enter the Promo Code
7) Tap 'Redeem'
That's it!The app will be downloaded and installed to your home screen.