ShareThis button is an all-in-one widget that lets users share content on the Web with friends via e-mail, Facebook, Twitter and many other popular channels.
Do make sure that you had registered an account at ShareThis before you proceed. If you haven't do so, you can sign it up at here.
Obtaining ShareThis's Property ID
You can get your Property ID from your Settings page.
Once you got your Property ID, you can paste it into Property ID setting.
That's it and you are good to go!