You need to have at least 200 reputation on the site you want to check in order this site can work
To get your user id, you need to authenticate yourself with Stack Exchange:
With authenticating yourself you only give us read access to your public data. We won't be able to post in your name or see any private data (Like your age or your email address).
Once you have authenticated yourself, select a site and click on this button to calculate your ranks: