Can't test Game Center on my device

[Visualazza]

Hi,

I have an ipod touch 4g and have just started adding Game Center to a game i'm working on.

This works fine in the simulator, but when i try to test on my device, the game isn't recognised by Game Center.

I've seen on other forums that i should be in sandbox mode, but i cant seem to enter it on my device.

Thanks,
Visualazza