The "templates" is where you should be placing all of the templates i.e. tpl files that are needed for the addon. Similar to how custom templates within themes can add to, or overwrite default Tiki templates, tpl files in the Addons templates are also added to the Tiki Smarty search path for template retrieval. Theme templates overwrite Addon templates which in turn overwrite default Tiki templates.
Subfolders should be created analogously to the Tiki default "templates" folder for features such as "activity" for custom Activity Stream templates.