Scroll to the bottom of the page and click on "contact." Or simply click here:
https://www.wgt.com/help-request.aspx
By the way, the only recent advertisement that I saw for free balls with purchase required a purchase of 2,000 credits. In fact, that's WGT's standard promotion.
In the almost 3 years I've been on here, I've never seen a promotion in which a person got free balls for a purchase of 500 credits. Are you certain you read it properly?