The problem some folks had about the widget displaying an warning is due to running an old version of WP.

Here’s my fix for the 2.0.1:

Change:
add_action(’widgets_init’, ‘widget_archive_drop_init’);
To:
add_action(’plugins_loaded’, ‘widget_archive_drop_init’);

Them change:
wp_get_archives(”type=monthly&show_post_count=$c&format=option”);

To:
get_archives(”monthly”,””,”option”,$c);

Hope it helps you.
Nice extension but poor support.