comment numbering?

I’m using Greg’s Threaded Comment Numbering plugin, which would automatically number comments if I only knew where to put the line of code. Could you give me some idea, please? Here are his instructions:

If you are using the default WordPress theme, or another theme which already supports the new comments features but does not use its own callback function, just replace wp_list_comments() in your theme’s comments.php file with wp_list_comments('callback=gtcn_basic_callback'). (See below for a note on safe wrapping of plugin-dependent function calls.)

The plugin includes some basic styling information suitable for the default theme, but if you’d rather style numbers yourself, you can disable this feature in the plugin’s settings page and provide your own within your template’s CSS file.