It only asked for my Amazon order number. I did it online, under the "Amazon Rebate Center." I was very impressed with how easy they make it to redeem rebates.
Now, I only wish I didn't receive a defective MPB (my MBP has the infamous high-pitch ringing and screen whine/buzz that all the MBP support forums are talking about). I already sent it into Apple Care for repair. I had it exactly 48 hours. Oh well. It's not Amazon's fault.