After upgrading to K2 5.1, you may not see this section on the K2 Management site:
This section is located in the following area in K2 5.0: K2 Management > Integration > API Configuration.
In K2 5.1, the API Administrator rights are part of the Server Rights and can no longer be separately configured. The API Administrators section has been removed.
To give the user API Administrator rights, please give that user Admin rights under the following section: Workflow Server > Server Rights.