Welcome to the Magix user to user forums, so not a contact for any Magix support staff and no-one here can make any changes to your account details, except you, and definitely nothing at all about any resetting of computer registrations!
In order to change/reset your email, go to your Magix account, log in using your old email address (it doesn't matter if that address is no longer in use, hopefully you can remember your password), go to User data and, on the page that opens, towards the bottom, click on "Change details". Follow all the instructions you then see.
Whilst still logged in, go back to the initial page and go to My products. Find the listing for your installation of ACL and check how many activations are recorded there. Deactivate any you no longer need.
Log out of your account, wait a few minutes and then try again to register your product.
If you cannot, for any reason, log into your account you need to email email@example.com, giving as much detail as you can about your old account and ask for their help in changing your email address and resetting your activation/registration count.