Purchase at least one qualifying FREE app offered in the Amazon.com Appstore for Android in the webstore or on an Android device. It will be displayed that it qualifies if it is qualifies:

You will then receive an email on how to redeem your credit. Limit one per customer. Offer is valid through June 30, 2012 and you must redeem the credit by July 31, 2012.

Want more free tunes? Check out these past free music download posts!