Where do I put the code please?

Can someone have a look here :

http://www.s2member.com/forums/topic/uri-restriction-for-buddypress-profile/

and please guide me where to put the following code:

$displayed_username = str_replace("/members/", "", $_SERVER["REQUEST_URI);
$slash = strpos($displayed_username, "/");
$displayed_username = substr_replace($displayed_username, "", $slash);

If I put the above code in wp-content/plugins/s2member/includes/classes/ruris.inc.php my website goes blank white!!

Thank you very much for the help..