you have to add the editable region to both the design template and the theme template.
Open the design template, select the area to convert to an editable region, and go to Insert -> Tempalte Objects -> Editable region.
then open the Templates/themes folder and open the theme template file.
and repeat adding the new editable region. make sure to use exactly the same name (case too).
The theme template is really nothing more than a list of all the editable regions on the page.