I started writing Illustrator plugins using the Adobe SDK in 1995. Beginning with AI 16 (CS6), the SDK dropped their ADM, with support for UI elements, and users needed to use a third-party solution. I didn't find any of the alternative methods at all intuitive or sufficienetly documented, and went with CORE, free from Hot Door (makers of CADtools).
http://hotdoorcore.com
You can download the libraries and compile the sample projects to see how it works. In addition to its built-in cross-platform UI support, I found it much easier to work with than Adobe's SDK, although it's possible to build plugins using parts of both CORE and the SDK. The downside is that CORE is updated sometime after each Illustrator release, so there's a lag between your updates and everybody else's.