Quickest and easiest solution would be to just take it back to the store you bought it from and ask for a replacement. (Still have your receipt, I hope)
Other than that, you can contact Hasbro and shoot their support section an email about it. I don't think they send replacement parts for Transformers, but they will often send you a replacement product. Depending on the price point of the item in question, they will send you a pre-paid mailer to send the item back (usually for items around the voyager price range or higher, but sometimes they don't require voyagers to be mailed back).
The problem is that you aren't guaranteed to get the same item back. Their policy is to send you a replacement Transformer product of equal value. But the Ark set seems to be in a price set of it's own, so I would assume that getting another one back is pretty good (but don't hold me to that).
I hope that helps.
