Download the plugin archive and expand it (you've likely already done this).
Put the 'CommentTweets' folder into your wp-content/plugins/ directory.
Go to the Plugins page in your WordPress Administration area and click 'Activate' for 'CommentTweets'.
Go to Settings -> CommentTweets and write your twitter account details or go to twitter.com, make a new twitter account for this plugin and then go to Settings->CommentTweets and use this account for the plugin.
Requires: 2.7 or higher Compatible up to: 2.7 Last Updated: 2009-2-24 Downloads: 1,213