It appears there was previously an extension for MMC in Wix2 advising the xmlns http://schemas.microsoft.com/wix/MmcExtension - attempting to read this URL results in an error page currently.
I cannot seem to locate equivalent for Wix3.6, is this deliberately (re)moved, or just missing?
It was removed, you have to manually create the registry keys. This link has a good summation of what you need to do.
The old extension was just a wrapper for the code included in the link above. I'm not sure why they took it out.