How to modify the "single page loop?"

[keener]

I want to use a related posts plugin with the Corporate Theme, and have the location and styling of the related posts links up to me. I figured on doing this by adding code similar to the following into the theme's single.php file

if ( function_exists('related_posts') && is_single() )  { 
?>	
<div class="postpost">
	<h4>Related Posts:</h4>
	<?php related_posts(); ?>

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	<h4>Bookmark and Share:</h4>
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</div>
<?php }

The problem is that I cannot find a single.php file. I looked for a theloop file to see if it took place of the single file and did not see it either. Basically I cannot find where in the code I need to add the above to get it to show for single blog posts.