Did you download and install mz3Tool already? If not please do so preferably now:
Download mz3Tool für Windows
Download mz3Tool für macOS
mz3Tool supports you in preparing your first Book “in not time”.
First you have to enter the license key, though. Your best option is to copy the key from the confirmation email you should have received by now and open the “New Project or License Window” and click the “Enter License Key” button. The key can be pasted (or dragged) into the opening license key field.