How do I authenticate to the GoodData API?

I am trying to authenticate / login with the GoodData API but I am having trouble. On the GoodData API documentation site (docs.gooddata.apiary.io) it says to POST to:

URL:

https://secure.gooddata.com/gdc/account/login

Headers:

Accept: application/json
Content-Type: application/json

JSON Body:

{"postUserLogin":{"login":"user@company.com","password":"YourPassword","remember":1}}

I get the response "Private API, please provide private hash." Any ideas?