As opposed to dotnetforab, this AmiBroker .NET SDK is absolutely free (as in free bear) and an open source (Apache License 2.0). It's just a port of the official C++ based ADK to .NET / C#. If you want build something on top of it, it's recommended to read the official C++ ADK manual first. You will need VS 2010 or VS 2012 to open the project; in project settings you can downgrade .NET framework to a lower version without re-factoring needed.