After downloading the product, I put in the serial number emailed to me but it wouldn't accept it. I went online to register but it said it was registered already. I can't access the program.
I don't understand. You registered as a user when you purchased the unnamed product unless you were already registered. This means that you have an account.
You activate the product with the serial number and your email address (username for your account) when you first install and open the product and it will be automatically registered to your account. You do not need to go online to register the product.
Did you purchase the product from Magix on Magix.com?
Which product and what, exactly, did you do and what, exactly, was the message and where in the process did it occur?