How can I download the Amazon Linux AMI

I'd like to build upon the Amazon Linux AMI. How can I download one of their AMI images using ec2-download-bundle?

I tried

ec2-download-bundle -b amzn-ami-us-east-1/amzn-ami-pv-2012.03.1.x86_64.manifest.xml <auth>

But I don't think that is correct.