Anyone who has played MC2 with an iOS 7+ device might have noticed that the game gets stuck when you try to type anything. Because of this keyboard bug, it was impossible to play online.
HOWEVER, my brother found that the Gameloft Live! in-game screen uses the iOS 6 keyboard, which functions as normal. And that's how you can play online.
Open the app (sounds won't work so close it and open it again to fix that), and go to online multiplayer. Then tap "new account". Gameloft Live! will pop up. Here, log in with your account. Then close Gameloft Live!, close (kill) the app, and open it again. Now when you tap online you will automatically log in.