WordPress.org

Manual Installation

Extract all files from the ZIP file, making sure to keep the file/folder structure intact, and then upload it to /wp-content/plugins/.

Automated Installation

Visit Plugins -> Add New in your admin area and search for this plugin. Click "Install".

See Also: "Installing Plugins" article on the WP Codex

Plugin Usage

Visit Settings -> Twitter Tools: bit.ly and fill in your login and API key.