I've already tried to activate my product code and it always says that someone has already registered it with another email. Why does it happen? I've already changed the product and that says the same all of the time.
Did you buy the product "new" or second-hand? If new there is no reason why activation should not work. If second-hand/second-user the original owner will need to contact Magix via email and ask for his registration to be cancelled. Once this is done you should be able to activate your copy.