<!--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
return $content . "\n\n<!--adsense-->";
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.