Download the plugin archive and expand it (you've likely already done this).
Put the 'wp-twitip-id' folder and all files inside into your wp-content/plugins/ directory.
Go to the Plugins page in your WordPress Administration area and click 'Activate' for WP Twitip ID.
Go to the WP Twitip ID Options page (Settings > WP Twitip ID) to set where you want the field to appear (defaults to after the url field) and add your chosen anchor text or image url