For AWS you will need to generate Access keys for the IAM user you want to use along with a suitable Role to grant permissions.
If you need assistance creating an AWS user and the required policy please refer to AWS Identity and Access Management Documentation .
In the EC2 console navigate to IAM -> Users -> Select a user -> Security Credentials -> Create Access Key
Save the Access Key and Secret Key
In Scalr in the “Add Credentials” dialogue select AWS, Enter a name and the two keys and save.