Mixcraft seems to accept my registration, but remains in trial mode.

Some versions of Windows security features/restrictions, do not allow many software products to write to the operating system registry. This causes the registration process to fail.

• Right click the Mixcraft icon and choose properties.
• From the resulting window, go to the compatibility tab.
• Check the box that says to run as an administrator.

Now close the box, run Mixcraft and register again.