WordPress.org

1. Upload the plugin to the /wp-content/plugins/ directory
2. Activate the plugin through the 'Plugins' menu in WordPress
3. Eat cake

Note: If you're running on WP e-Commerce 3.8.7.5 or below, make the required changes to your theme file - see here for details:

• http://code.google.com/p/wp-e-commerce/source/detail?r=983
• http://code.google.com/p/wp-e-commerce/source/detail?r=818