Now that you have created your Application and configured authentication, you can subscribe to the Provisioning API. This will give you access to the Account, Extension and User APIs.
Click the API Directory link in the left-hand navigation menu.
Locate the "Provisioning" API and click "Go".
From the "My Applications" drop-down in the right-hand corner of the page, select your Application and click "Subscribe":
By subscribing to the Provisioning API, your Application now has the correct privileges to use the Account, Extension and User APIs. Look at the code snippets for each API to learn how to use them.