WordPress.org

1. Download replyMail from WordPress Plugin Directory, and unzip it.
2. Upload "replymail" folder to your WordPress's plugins directory (always "/your-wordpress-directory/wp-content/plugins/").
3. Activate the plugin through the 'Plugins' menu in your WordPress.
4. Now, go to 'Setting' -> 'replyMail Setting', change the options.

Usage

1. Make sure you enable the threaded comment system in your WordPress, Go to 'Settings' -> 'Discussion Settings', select 'Enable threaded (nested) comments', and then 'Save Changes'
2. Make sure your theme support WordPress's threaded system. (your can migrating your theme to 2.7)