This offer is displayed on my Kindle with Special Offers (which is just an Kindle with Ads, lately), but is not limited to those with a KSO or any Kindle device. Instead, you can either order this one direct from your KSO or just go online to AmazonLocal and grab it there:
Free Voucher for $3 to Spend at Amazon MP3
You will need one-click turned on for your Amazon account AND you must have a credit card registered for one-click (even though you won't be charged). You can turn this off and remove the card (do this by choosing to supply a new payment type, then just clicking on your address without entering any other info), once you've signed up. Sign-up for this offer expires at midnight (PT) or when supplies are exhausted (check other cities if yours is out).
You'll receive the promo code on your KSO home page and can retrieve it from the Vouchers link at AmazonLocal (you should also get an email telling you when it is ready) and will need to load the credit onto your account (which you can redeem HERE), just like a gift card, before going shopping. Once you have entered the promotional code(s), you have until midnight, July 11, 2012 to complete your purchases. I'd recommend that you apply the credit right away, since it works on any MP3 purchase - then you can shop on the web OR from your Kindle Fire and it will be used automatically.
If you were holding off on any of yesterday's bargain albums, now's the time to grab one (the credit will buy an entire Soundtrack!).