The API is included in Extreme Management Center v7.0.
For previous releases, you must download and install a plugin that is kind of hidden. You can find it in the Extranet under SDK Connect:
https://extranet.extremenetworks.com/downloads/Pages/OneFabricConnect.aspx
There, under the Partner Resources tab you will find several different versions of the plugin. Which one to use depends on the release of NetSight (now known as Extreme Management Center)