To register Mixcraft, open the software and click the "Help" menu, followed by clicking "Enter Registration Code".
Skip #1 (buy a code on the Internet), if you have already purchased a registration. Enter your Registration ID in #3 and Registration Code in #4. Click the "Register" button in #3.
If you are unable to locate your registration information, you may request it at our automatic code lookup.
Here's a video on how to register: How to Register Mixcraft.