To download individual artifacts via the API you first need to get the artifact URL. This is documented here: https://circleci.com/docs/api/v1-reference/#build-artifacts
Also note that the value of circle-token should be set to a valid API token that you create from your account page: https://circleci.com/account/api
A shortcut for downloading all artifacts for a build is described here: Download all artifacts for a build on CircleCI