"Phoenix assumes that our PostgreSQL database will have a 'postgres' user account with the correct permissions and a password of 'postgres'. If that isn't the case, please see the instructions for the ecto.create mix task."

Alternatively, just edit the .exs files in config/.

On NetBSD the default PostgreSQL user is called pgsql. Rather than add yet another user to PostgreSQL you can just edit the Phoenix config files. I pulled out the username and password details from config/dev.exs, config/prod.secret.exs and config/test.exs and added this to the top of config/config.exs instead (just after use Mix.Config):

# Configures the database
config :hello_phoenix, HelloPhoenix.Repo, adaptor: Ecto.Adaptors.Postgres,
  username: "pgsql",
  password: "pgsql"

This, for example, is what remained in config/test.exs:

# Configure your database
config :hello_phoenix, HelloPhoenix.Repo,
  adapter: Ecto.Adapters.Postgres,
  database: "hello_phoenix_test",
  hostname: "localhost",
  pool: Ecto.Adapters.SQL.Sandbox