Category Page Plug-in

First, I would like to thank you for the very user friendly theme that allows idiots like me to make something that looks really nice.

I would like to add the Category Page plug-in, which requires a small code addition.

They say we need to add the code below, based on Kubrich

1) Open archive.php and find lines #8-9:

<h2 class="pagetitle">Archive for the "" Category</h2>

2) Change them into:

if( function_exists(page2cat_output)): page2cat_output($cat); else: ?>
<h2 class="pagetitle">Archive for the "" Category</h2>

(Here is a link to their site http://pixline.net/2008/04/category-page-25-howto-2/en/)

Could you tell me where I would add that code, with your theme?