The Autodesk Revit API Software Development Kit (SDK) is installed from the Tools and Utilities section of the Autodesk Revit installation. In the SDK, there are example files that will help you get a better understanding of the API and its use. Each example file has a sample .addin manifest file with the information that you will need to edit and place into the appropriate folder, which Autodesk Revit will access on launch.
RevitAPI.chm is the Autodesk Revit API reference documentation help file, included with the SDK package in the \Revit 2015 SDK\ folder.
For more information:
Resources:
www.autodesk.com/adn - Autodesk Developer Network home (ADN)
www.autodesk.com/developrevit - Autodesk Revit development resources
http://forums.autodesk.com/t5/Revit-API/bd-p/160 - Revit API discussion group
http://thebuildingcoder.typepad.com/blog/ - The Building Coder, an ADN blog dedicated to Revit coding
http://bimapps.typepad.com/ - Bim Apps, a blog dedicated to BIM applications