You can obtain an API key via home.mendix.com.
In home.mendix.com go to the project you want to access via the Story or Feedback API (or any future API accessing projects), and go to the settings of the project (click the cogwheel in the top-right of the screen). Now click the fourth tab, called Project API keys. You can generate a key there.
Please let us know what you think of these Project API's!