Are you talking about your blog page?….Meaning and ad shows up after the first post and then your other posts show up after that? I’m not sure how to do that exactly….I’m guessing it’s possible, but it’ll most likely require some type of custom code to implement.
Or, are you asking how to add it after the post on the actual blog post itself? If so, you can add it to the “After Entry Header” widget area on your widgets page since it’s already included in the latest version of the theme.