Solved: See below.
So, I decided to buy the optional HEVC encoder for 5 USD. I gave Magix my credit card info and got an Activation Code, as I have before with other add-ons. I copied and pasted the Code from Magix's email to me into the field for the codec, as I have for other add-ons.. It phoned home (on the Internet) and then reported that the Code was invalid. Error -14. But it doesn't allow me to re-enter the code! I sent a request to Support. In the meantime, any ideas how to fix? (And a warning to anyone else who might try this codec.)
MEP Plus 2016, 15.0.0.77; Win 7 Home, 64-bit, SP1 + patches