Yoast Breadcrumbs Plugin installation

I’ve been trying to install Yoast Breadcrumbs. The plugin site says:

Add the following code to your theme’s template(s):

< <?php if ( function_exists(‘yoast_breadcrumb’) ) {<
< yoast_breadcrumb(‘<p id=”breadcrumbs”>’,'</p>’); <
<} ?> <

I use a Woo theme. I added this to Themes.php. Crashed. admin-themes.php. Crashed. Am I supposed to create a new .php file in \themes\includes? I don’t understand where I am supposed to put this piece of code.