WordPress.com Stats entry in support docs > http://en.support.wordpress.com/stats/#additional-info
Note we cannot install Google Analytics on free hosted WordPress.com blogs. There is no upgrade here aside from your blog being accepted into VIP hosting that allows you to run Google Analytics.
This is a multi-user blogging platform. Our blogs share a common architecture. We bloggers cannot access metadata on free hosted WordPress.com blogs and install any tracking code, we have no FTP access and we cannot install any plugins.
FTP access, and plugins are available only for VIP bloggers http://vip.wordpress.com/our-services/ or for those with WordPress.ORG installs. http://support.wordpress.com/com-vs-org/
You will need to hire a web host http://wordpress.org/hosting/ and set you your own WordPress.org software install to install any plugins.