WordPress.org

Copy the 'twitter-widget.php' file to your WordPress '/wp-content/plugins/widgets' folder.

1. Activate the 'Twitter' plugin in your WordPress admin 'Plugins'
2. Go to 'Presentation / Widgets' in your WordPress admin area.
3. Drag the 'Twitter' widget to your sidebar.
4. Configure the options:
   1. Account: your Twitter account name, required
   2. Title: the heading you want to appear above your Twitters in the sidebar, defaults to 'Twitter Updates'
   3. Show: the number of Twitters shown, defaults to 5

WordPress 2.0.x and 2.1.x users

You guys should also be able use this widget if you install the Widgets Plugin. I tested it briefly and it appeared to work.

WordPress 2.5 users

Beware, if you leave the 'twitter-widget.php' file in the 'plugins/widgets' folder you will erroneously be prompted to 'upgrade' it on the 'Plugins' admin screen. I recommend moving the 'twitter-widget.php' file into the 'plugins' folder.