The Install doc is available, but the ADK itself and the rest of the documentation cannot be found.
Look in IntegrationServer\packages\WmART\code\jars\wmart.jar under your webMethods installation directory. This is very poorly documented - I had to just randomly open JAR files until I found the right one.
The webMethods ADK version 6 is available upon request from webMethods and includes full documentation.