For Ubuntu instances:
chmod 600 ec2-keypair.pem
ssh -v -i ec2-keypair.pem ubuntu@ec2-174-129-185-190.compute-1.amazonaws.com
For other instances, you might have to use ec2-user instead of ubuntu.
Most EC2 Linux images I’ve used only have the root user created by default.
See also: http://www.youtube.com/watch?v=WBro0TEAd7g