Install from the disc, if you have one, or download MMM 2014 from the Magix website - go to magix.com and click the My Account button
Login using your email address and password you used to register the software, click My products.
This will take your list of registered products and you will find a download link and your serial number(s).
Install and register - if you have exceeded the number of allowed activations then you need to contact support at magix.com to have the activation counter reset.
Once this is done copy any projects, audio files etc from the old computer to the new - keep the folder structure the same to avoid getting opo ups asking to locate the files required in the project(s)