help assembling an SQL statement in a plugin

i asked how to restrict nearfutureposts to categories. and the very generous (albeit busy) writer came back and offered this snippet:

AND ".$wpdb->post2cat."category_id in (1,2,3)

… which should be added to the main function in the plugin… but sadly i don’t know where to paste it! (many tries turned up SQL errors) this is the original line…

$content = preg_replace("/AND post_date_gmt <= '(d{4})-(d{2})-(d{2}) (d{2}):(d{2}):(d{2})'/", "AND post_date_gmt <= '".$nearfuture."'", $content);

where should the snippet (at top of message) go?

nearfutureposts plugin homepage where this came from:
http://blog.idahocline.info/2005/07/04/near-future-posts-plugin/