Windows 10 Technical Preview isn't getting shipped with Windows Media Center. Till now it is not even certain whether the Media Center is going to ship with the new OS, but you can add Windows Media Center to Windows 10. But to make it work you have to take a certain number of steps.
DON'T USE THE UPDATE :The first thing to keep in mind is you should not try a key which is being used on another device. Else it will not get activated though apparently it may not seem to be a big issue when you are just experimenting with the OS. But at the same time you need to remember that once you are using this method you will not get the updated builds as in that case the Windows will work on the assumption that you are working on Windows 8.1.
So in order to get the Media Center working on Windows 10 you need to go to the search box from the start menu and type "Add Features to Windows" and then when the Add Features to Windows Technical Preview will be pop up, you have to click on it.
You will be then asked to choose the option between using a product key or to purchase one. If you aren't particular about purchasing one, your Windows 8 Media Center key will work in here.
So just Click on "I already have a product key" and enter your key.
At this point it is difficult to say whether Microsoft will want your key to be used and test it. Therefore you might have to take up a small risk but when you click "next" it is most likely to validate the key. Now you are ready to add the Media Center. Just Click Add features to round it up.
Hopefully this will work for you for the few upcoming builds of the OS and right now in the final version. Along with a couple of reboots Windows Media Center will start up and be running. And you can continue enjoying Media Centre.