Follow these steps to apply for a Bumper or case.
1)Go to App Store and Search for "Case Bumper". You should be able to see the app.
2)Download and Launch the app on your iPhone 4 and sign in using your iTunes Store account or Apple ID.
3)Select your Bumper or case.
Visit here for more details.
For iPhone 4 purchases made before July 23, 2010, you must apply no later than August 22, 2010; otherwise, you must apply within 30 days of your iPhone 4 purchase. To qualify for this program, you must purchase your iPhone 4 by September 30, 2010.