Create a EC2 Key Pair

http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-key-pairs.html#having-ec2-create-your-key-pair

Open the Amazon EC2 console: https://console.aws.amazon.com/ec2/

Navigation pane > Network & Security > Key Pairs

Create Key Pair

Enter a name for the key pair and press Create

You will be prompted to save the private key file on your local machine, save it in a safe place.

Creating Instance That Will Use Key Pair

More Options > Security > Virtual Machine Permissions > EC2 Key Pair

Existing Instance To Use Key Pair

Configuration > Instances > gear settings button > > EC2 Key Pair

Login Using SSH Client

Host: The instance URL

Login: ec2-user

Password: [leave blank, even if prompted]

Private Key: Your .pem file