Paste the RSA private key of the EC2 key pair. If you are new to EC2 and have no idea what this is, you can just generate one (if you do so, you should save the key into a text file, as you will need it if you want to login to the EC2 instances via ssh.)

An RSA private key for EC2 is a text that starts with "-----BEGIN RSA PRIVATE KEY-----". You normally generate it via using either the AWS Management Console, the "ec2-add-keypair" tool or a GUI tool like ElastiFox.