To redownload your product go to your Magix account, log in, go to My products, find the listing for MMS Platinum 18. There will be links to the installer as well as showing your needed serial key. It will also show any existing activations previously made of the software. If you no longer need any/all of those listed you should deactivate them whilst in your account. This should mean you avoid the problem of attempting to install and activate the software too many times! (You can have a maximum of two simultaneous activations in operation.)
Once the installer is downloaded, simply double-click it and allow it to run. It is worth checking every screen that is shown as it first begins; there are various user options which, if not specifically checked by you, may result in the default installation of other software you may not actually want! IOW, the advice is Don't just click on every screen you see to accept those default settings! Read carefully and make your choices based on what you want to see installed and where!