If you are one of those who have joined the Windows Insider program and installed trying out Windows 10 Technical Preview (TP), then you should not add Media Center product key. If you add then System Properties will show it as “Windows 8.1 Pro with Media Center” and will say Windows is not activated.
And not only this, using a purchased product key will also prevent you from getting updates and future builds for Windows 10 Technical Preview.
Microsoft in the Answers Community forum had replied in one of the queries:
We apologize for not catching this before we launched Tech Preview; we did not intend for you to be able to use a purchased product key on a preview build.
We do not recommend adding Media Center to Windows 10 Technical Preview as using a purchased product key will also prevent you from getting updates and future builds for Windows 10 Technical Preview.
To make sure you get updates for Tech Preview, we recommend that you download the ISO file and perform an in-place upgrade to re-install Windows 10 Technical Preview.