As far as I know, only Etherdelta is the exchange where ATS is currently listed. You can register your MEW to Etherdelta and make an exchange on it. However, I don't recommend to exchange now since the price is really low from what they have said before the airdrop (ideally 0.15$)/ I would suggest to hold t first and wait until it pumps up on the market. Additionally, devs are continously adding better exchanges. I don't prefer Etherdelta actualy