When starting the program I am taken to the registration page, my serial number and the correct email is shown, if I click "already registered", the program ends. If I submit registration information, then the program says "Already registered" and then the program ends. In either case I can get no further.
You will need to go to your Magix account where you originally registered your copy of Movie Edit Pro and de-register it. You may also have to generate a new activation code while you are there. Then restart MEP and when the box comes up again put in the new code.
If you can't do that for any reason you will have to get in contact with Magix using the infoservice@magix.net email address and ask them how to proceed.