I didn't understand this at first, so posting here in case anyone else has same problem.
On left side of Olark for WP page in settings, in WordPress admin, it asks for Olark Account ID. I believe this is what you need:
Log in to your Olark Account and then click on the install tab. There it says:
Place Olark on your site!
Your site ID is (in case we ask you, or a plugin requires it):