If you've downloaded software that requires a product key (or license / serial number / activation code), the product key can be found on your order receipt.
To view your product key:
Sign in to your WebStore (if you are not signed in already).
Click the Your Account/Orders link near the top-right of the page.
On the Orders & Downloads tab, click the View Details button next to the product you need the key for. Details about that product, including your product key, are displayed.
If you are having trouble locating your product key, contact support by clicking the Contact Us link at the bottom of the Help page.