Why Isn’t Imessage Working On My Ipod?

I have imessage on my ipod because of IOS 5, and My mum also has an iphone on IOS 5, but when i try to send it a message, it just says; THIS USER IS NOT REGISTERED WITH IMESSAGE.
I don’t get it, All the contacts are filled out, there both on IOS 5, and they both have Wi-Fi.
And the basic guide doesn’t say anything about imessage, WHAT DO I DO?