This feature requires a paid subscription.
First go to "My Publications"
and then go to the settings of the magazine:
Scroll to "Analytics" and add the Analytics Property Code to the magazine (for example "UA-27868640-1") and hit "Save". You can use a newly created property code or one of an existing magazine. In the second case the statistics will be counted to the one of the website. You can also use one code for multiple magazines.
If you need more information on how to generate the Google Tracking code please refer to the documentation and FAQ's of Google: https://support.google.com/analytics/?hl=en#topic=3544906