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These two rows must be in function.php in the theme for the plugin to work:

add_theme_support( 'post-thumbnails' );

set_post_thumbnail_size( form_option('thumbnail_size_w&&echo=false'), form_option('thumbnail_size_h&&echo=false'), true );

You'll also need to add thumbnails to the posts you want visible in the list.

After that, all you need to do is to add the hook [categorythumbnaillist 3] (where 3 is a categoy id) to your post.