To be able to use your Amazon EC2 user's key/pair to list your instances you need to ensure you have assigned the correct permissions to the user that the key/pair is assigned to.
You can either create a new group with the correct permissions for controlling EC2 instances or you can create a single user policy. For the purpose of this article we'll add one of the default policies to our user.
- Click on your account name at the top right of the Amazon AWS console and click on "Security Credentials". Now click on "Users" on the left.
- Click on your user in the list and information related to that user will load just below.
- Click on the "Permissions" tab and then click "Attach Policy".
- Choose "Amazon EC2 Read Only Access". The read only access will only allow us to list your instances, which is all we need!
More experienced AWS users can create an individual user policy and/or group with very specific list permissions for their user using Amazon's custom policy creator. You can read more about this at http://docs.aws.amazon.com/IAM/latest/UserGuide/ManagingPolicies.html.