When you follow the link in Scrivener to the Amazon KindleGen page, that will redirect you to the download page for Kindle Previewer. Although KindleGen is no longer a separate download, there is an updated version of it bundled deep in Kindle Previewer. On my machine, the directory for it was:
C:\Users\devin\AppData\Local\Amazon\Kindle Previewer 3\lib\fc\bin\KindleGen.exe
I was able to click the “Choose” button and provide that path, and Scrivener RC11 then showed the normal options:
[attachment=0]mobicompileoptions.jpg[/attachment]