You skipped the 3rd step of the installation process. Try again. :)
Navigate to Settings -> @Reply with preview.
Either you have disabled JavaScript in your browser or your WordPress theme is not using the default id for the comments <textarea> (which is comment). In the later case, check your comments.php file to find your textarea id, then go set that in the Options for this plugin.
Either you have disabled JavaScript in your browser or your WordPress theme is not using the default class for the comments text's encapsulating <div> (which is comment-text). In the later case, check your comments.php file to find the id of the div containing the comments' text, then go set that in the Options for this plugin.
In addition, you might have to add position:relative; to the stylesheet definition of the DOM-elements holding the comments in order to make the preview positioning work. You can also use the custon CSS section in the plugin's settings (eg. div.comment-text { position:relative; }).
A plugin that filters the default WordPress comments functions, may prevent @ Reply from working. For example, WP_Identicon does this, but it also has an alternate setup for "advanced users" that doesn't conflict with @ Reply.
Please provide some feedback @ the forums




