I am using Genesis Widget Areas Generator plugin. After activating this plugin you will get a new menu “Widget Generator” at left side panel. I am creating the two new widget areas for single post.
One is “Before Entry Content”.
Another one is “After Entry Content”.
Now navigate to Appearance -> Widgets page and you will get two new sidebars there. Drag&drop the Text widget there and paste your Ad code. Your Ad will show on every single post.