You should copy all files from:
includes/languages/english/html_includes

to your templates and overrides directory:
includes/languages/english/html_includes/your_template_dir

Then set them to 777 if you want to edit them from your Zen Cart Admin ...

NOTE: if you have already done that do not do it again, as you will overwrite your changes ...

When editing them in the Tools ... Define Page Editor ... you should see something similar to:
Editing file:
/home/your_user_name/public_html/your_directory_name/includes/languages/english/html_includes/your_template_dir/define_main_page.php

If should not you see:
/home/your_user_name/public_html/your_directory_name/includes/languages/english/html_includes/classic/define_main_page.php

if you do, then you have not made templates and overrides but are using the default template ...