First, you must be on version 3.46 or higher to get Webinar Scores.
If you are upgrading to a newer version which did not previously calculate the Webinar score, or if you ran your initial GoToWebinar sync prior to inserting your Webinar Score values in your Custom Settings, you can still get your current scores by executing the following code from the Developer Console
Go to your name or to Setup in the upper right navigation
Select "Developer Console" > Debug > Open Execute Anonymous Window
> Insert the following string:
Id bactchJob = Database.executeBatch(new CalculateDefaultLeadScore(),200);
Finally, click on the "Execute" button
NOTE: This is a one time event. Going forward, as your users Register for and Attend Webinars, the Webinar Score will calculate automatically