On EC2 all the Ubuntu AMI's should be connected to like so:

$ ssh -i your-ssh-key.pem [email protected]

So you connect as the 'ubuntu' user using the ssh key you injected into the instance at launch (not a password). Once authenticated, you can issue password-less root commands with sudo.

(note: your ssh key might be .priv, or .pem, or no extension, this is just an example)

Here are also some handy resources about connecting to EC2 instances:

Launching an instance and choosing or creating a new SSH keypair: http://docs.amazonwebservices.com/AWSEC2/latest/GettingStartedGuide/index.html?LaunchInstance.html

Connecting with your ssh keypair:
http://docs.amazonwebservices.com/AWSEC2/latest/GettingStartedGuide/index.html?ConnectToInstanceLinux.html

What they don't mention there is that you connect to Ubuntu instances with the 'ubuntu' account and not root.