Those instructions are for an actual pod where either you have access to the admin console or you can request for your pod admin to help create a service account for you.

For the Developer Sandbox, you can request for credentials via the welcome email (which you have already done). These are automatically provisioned for you so you do not need to access the admin console on the sandbox. You will then receive another email with your credentials package. Once you have obtained that, simply extract it and replace the keys produced by the generator with the keys from the package and revise the bot username in your configuration respectively.