You must have wither a Pro or API account to gain access to the API. If you signed up for an API-only plan, you should have been sent an API key and password via email. For Pro accounts, you must download the application and setup your account from within the software. Following that you can continue with these instructions.
To find or register for an API key, simply login to your existing account at http://support.chimprewriter.com/ and going to the API tab.
By using your registered email and password, you can find your API Key here: