2 possibilities: first the balls are in our inventory and you can find them by opening your edit equipment tab and looking at your available balls.
Second was that you responded to "purchase required" promotion which would go something like "if you come back and purchase $20 in credits we will give you a sleeve of balls".
I have never known WGT to offer a sleeve of balls to players who have left the game for some period of time as an incentive to return. Doesn't mean that it doesn't happen, just that I have not heard of it. But they are forever making the free balls with purchase offer.