I just purchased the codec for Magic Edit Pro 2016 Plus. I paid my money, was sent a confirmation which included the activation code, but I kept getting error messages
I'm afraid you should be a bit more precise with your information for us to help effectively but I'll have a guess.
Which codec?
If it is for HEVC then the Magix codec is for encoding (exporting to H265) only. You would have to also buy the decoding codec from the Microsoft store to allow HEVC files to play within the editor.
. . . . I just purchased the codec for Magic Edit Pro 2016 Plus . . . .
This is an old version of Movie Edit Pro, where did you purchase this from?
If purchased 'used' and are getting 'Error -24 registered to another email address' then the seller needs to de-activate the software before you can register it.
If from a retail store you may have a voucher code for redemption, the voucher code will be a combination of letters/numbers, a serial number begins with P3-, in which case try redeeming the voucher here.
If neither of the above what error message(s) are you getting?