WordPress.org

From wordpress admin screen

1. Choose 'Plugins > Add New'.
2. Search for 'twitcategory' and install!

From this page

1. Click 'Download'
2. Upload twitcategory.php to your /wp-content/plugins directory.

Remember to:

1. Activate the plugin through the Plugins menu in WordPress Admin (you may need to be using a wordpress administrator account for this action). Please note that TwitCategory will create a table in your wordpress database called 'twitcategory'. It will clobber any previous table with that name.
2. Fill in your twitter account information in the twitcategory configuration menu found in the Manage menu in WordPress Admin.
3. Rate and give feedback on http://wordpress.org/extend/plugins/twitcategory/