Displaying author name in each post

Hi. I’m using the GameWorld theme over at geekinitiative.com. I would like the author name to display in each post, and this is the first theme I’ve ever used that doesn’t automatically do that. I understand I need to edit code. I’ve found a few answers on this site telling me where to add the following line:

<p>Written by <?php the_author(); ?></p>

Unfortunately, editing index.php, single.php, and ‘the loop’ file and adding this line do not fix the problem. Pasting it into the custom CSS file does nothing. Which file, specifically, should I alter, and where should I add this line?

All I want to do is have the post automatically say “Written by AuthorName” where ‘AuthorName’ is the name of the author.

I’m really surprised this isn’t a WYSIWYG option in the theme or in WordPress, generally.