Login To your Plesk Control Panel and navigate to the domain in question.
Click Web Hosting Settings
Then tick the Click Web statistics check box, along with the check box next to /plesk-stat
Then click Password Protected Directories.
You will see an option for /plesk-stat, click this
You will then see a user name listed, click this and set a password.
That's it. Stats update daily, so once enabled, please allow 24hrs to Update. You can then use the below URL along with the username & password set above to access your stats