iMessage won't activate ??

Hello

I have just got my new iPad ' iPad 3'

I have tried to activate the iMessage but it won't active at all. I have tried to use my apple ID and it say ' the user name or the password was incorrect'. The apple ID works fine on the store but can't get it work with iMessage. I have reset all the setting and even I have download the iOS 6 twice and It did not work.

Need sum help plz.

iPad 3 wifi + 4G 'AT&T'

Thanks