Hello. Turns Twenty Twelve theme has a single.php file something different from other subjects. From what I read in the forum this issue is resolved. They have done as follows: content.php file copied and have called post-content.php, File sigle.php changed
<? Php get_template_part ('content', get_post_format ());?>
By this other
<? php get_template_part ('post-content', get_post_format ());?>
and have placed the AdSense code in the post-content.php
Anyway, I do not know where in the post-content.php file I have to put the adsense code to display below the post title. I hope I explained correctly.