I've been working on this for close to two hours. I've used SS many times before, but haven't needed to add a custom field. After an hour I got it to show a new HtmlEditorField, but there is no title set to the field when you look at it in the admin. It works and I can display the content on the page using both $Sidebar_Title and $Sidebar.
I have this and the TextField is showing what I have there, but the HtmlEditorField is not. P.S. this is in the Page class. Yes, i have gone through the tutorials as well about a dozen times :)
static $db = array(
'Sidebar_Title' => 'Text',
'Sidebar' => 'HTMLText'
);
function getCMSFields() {
$fields = parent::getCMSFields();
$fields->addFieldToTab('Root.Content.Main', new TextField('Sidebar Title'), 'Content');
$fields->addFieldToTab('Root.Content.Main', new HtmlEditorField('Sidebar'), 'Content');
return $fields;
}