how to add ad at single post page

Some help in Codex: Using_Images#Using_Images_in_Posts

or, how about a plugin?
http://www.acmetech.com/blog/adsense-deluxe/

<?php if ($single) { ?>
Put your ad code in here.
Put this whole thing where you want the post to appear.
This cannot be inside The Loop unless you use another plugin.
<?php } ?>

1. Link to an example post
2. Tell us exactly where you want the ad
3. We’ll tell you 🙂