This offer is only for those with a Kindle with Special Offers (including any Kindle Touch or Kindle Keyboard that opts in to Special Offers, which means you can add the offers, grab this one, then turn them back off!):
Get $3 off at Amazon Instant Video
Click on the offer, then click on the link on the offer page sign up and you'll get an email with the promotion code. Sign-up for this offer expires May 6, 2012.
You'll get an email (right away), a link to the Instant Video page, a promotion code and a link to the redemption page. This one works like a gift card and you need to apply the credit to your account first, before going shopping. Once you have entered the promotional code(s), you have until June 6, 2012 to complete your purchases. I'd recommend that you apply the credit right away, since it works on any video purchase - then you can shop on the web, from your Kindle Fire or your Roku and it will be used automatically.
You can use this credit on a rental or to purchase single items or entire television seasons. I'll probably use mine for the occasional 99 cent movie rental; if you have multiple KSO's, be sure you have them registered to different accounts before you try to claim this one or they'll all end up with the same code. With different accounts, each KSO can claim and use the code (three $1 rentals each should be enough to keep you in movies for the month, if you have 2 or 3 KSO's!).