To enable popup comments, choose the appropriate method for the version of WordPress you are using.

WordPress version 2.0 Default Theme

Inside header.php add this line:

<?php comments_popup_script(); ?>

above this line:

<?php wp_head(); ?>

For example:

<?php comments_popup_script(); ?>
<?php wp_head(); ?>
</head>

WordPress 2.0 Classic Theme

In the file header.php locate this line (line 21):

<?php //comments_popup_script(); // off by default ?>

And remove the first comment markers, changing it to this:

<?php comments_popup_script(); // off by default ?>

WordPress version 1.5 Default Theme

Inside header.php add this line:

<?php comments_popup_script(); // off by default ?>

below this line:

<?php wp_get_archives('type=monthly&format=link'); ?>

Example:

<?php wp_get_archives('type=monthly&format=link'); ?>
<?php comments_popup_script(); // off by default ?>
<?php wp_head(); ?>

WordPress version 1.5 Classic Theme

Inside header.php is this line:

<?php //comments_popup_script(); // off by default ?>

Change that to

<?php comments_popup_script(); // off by default ?>

WordPress version 1.2

Inside index.php is this line:

<?php //comments_popup_script(); // off by default ?>

Change that to

<?php comments_popup_script(); // off by default ?