Hi John,
Most features require a page context for editing to enforce page level edit permissions. The HTML Content feature is the only one where we have tried to make exceptions to that rule. I thought it was already implemented to support admins and content admins editing it without a page context but I was able to replicate this problem as Content Admin. I just fixed this in my copy so it will be fixed int he next release. But for now you can either edit as an admin or set the module edit roles to a specific role that the user is in. I would not set it to authenticated users since that would let anyone who can sign into the site edit once it is published on a page.
Best,
Joe