WordPress MU

I am going to test a popunder through ALL the blogs on the blog network. What file must I edit to make it every time someone visits a blog a popunder will appear.

Thank you

You can use the wp_footer hook to insert data to user blogs.

Here's an example:

<?php
function footer_stuff() {
?>
<!--START CUSTOM STUFF -->

<!--END CUSTOM STUFF-->
<?php
}
add_action('wp_footer', 'footer_stuff');
?>

Copy the above code to a file and rename it to something.php and place it into your mu-plugins folder.

Place anything between the START and END tags and it should be displayed in each blog.