To append <!--adsense--> to the end of EVERY post that doesn't already have it, you can use this little piece of code. I haven't tested it, but it should work. Put it in one of these:
1) Your theme's functions.php
2) The end of the adsense deluxe or wp ads plugin file
3) Into a blank PHP file, with a plugin header at the top of the file, and then activate it as a new plugin
<?php
function adsensify($content){
if (false!==strpos($content,'<!--adsense-->'))
return $content;
return $content . "\n\n<!--adsense-->";
}
add_filter('the_content','adsensify',5);
?>
Note that the <!--adsense--> thing won't show up when you edit posts--it gets appended right before the post gets displayed on your site.