Just like Ricks "plan" this hasn't been explained yet. They made sure though, everybody knows now Eugene had something to do with it and not Daryl. They didn't show Carl being bitten "getting him here".

Eugene probably rigged another ipod/plane contraption like he had done earlier when Dwight shot it down. That would be my guess anyway, although, who knows. I believe Carl was bitten when he was in the woods with Siddiq. They were killing the walkers and Carl had fallen on that dead deer with a few walkers on top of him. Unless they show some other flashback, I think that's the only time it could've happened.

@tipsymcstaggers said:

Eugene probably rigged another ipod/plane contraption like he had done earlier when Dwight shot it down. That would be my guess anyway, although, who knows. I believe Carl was bitten when he was in the woods with Siddiq. They were killing the walkers and Carl had fallen on that dead deer with a few walkers on top of him. Unless they show some other flashback, I think that's the only time it could've happened.

Yes, that is when he was bitten. I believe it has been confirmed. I think in the beginning of this mid season finale, Carl was writing a "good bye" letter to Rick.