Forgotten Email:
Critters could try to Private Message RocketSnail on Twitter, but due to security reasons, you might have to create a whole new Account, if you have forgotten your Email.
Forgotten Username:
Since Box Critters requires you to enter in your Email to log-in, all you must is log in with your Email that you used to create that Account, and your Username will be visible in-game. Looking to Change it or does it appear as Critter, check out those Help Pages for further information.
Box Critters Help 