Please note that the "ASP" and "PHP" menus shown above, ONLY appear after you have established the appropriate server model and SAVED your page with the extension of .asp or .php.
The menus are context sensitive and you will only see the one appropriate for your own server model. But you won't see them at all if Dreamweaver doesn't recognize the page format.
If you want to use an SSI in an HTML page, you'll have to add the code by hand and save your page using a dynamic extension, such as .shtml. Most browsers won't support an SSI unless you are using a dynamic server model or file extension.