What exactly did you do in your site.ini? Here an excerpt form the site.ini:
[ExtensionSettings]
ExtensionDirectory=extension
# A list of active extensions, add new ones to activate them
# The extension itself will then have it's settings directory read (if any)
# and any extra configurability is done automatically.
ActiveExtensions[]
# Same as ActiveExtensions but will be loaded after the siteaccess is chosen.
# This allows for activating different extensions on different siteaccesses
ActiveAccessExtensions[]
The way I read this you have to specify the html editor in the ActiveAccessExtensions in the admin site access, not in the ActiveExtensions. Does that work?