The WordPress.com Stats service is listed on the PLUGINS page, down towards the bottom. Once you have activated it, you will have access to a SETTINGS page with directions on how to set up the service.
This plugin uses the stats software from WordPress.com, so you will need to make an account with their service for each of your blogs. Once this is done, you can log in to the stats from your DASHBOARD and see how many hits, when, and where from.