Best answer by Ed_Boost
It sounds like you have a smart meter. If that's the case, you don't need to register your new electricity card.
The electricity card should say 'Electricity Payment Card', or 'Energy Payment Card', with a long, 19 digit card number on the front.
Just go and top it up at the shops and it'll remotely move the credit onto the meter.
If you have an electricity key, which looks like a USB stick, then you'll need to get in contact with us at firstname.lastname@example.org, or via Facebook or Twitter, so we can get the right one out to you.