The steps below are for Start8 but are the same for MM.

Right click on the Start8 button and select "Configure"
(If you no longer have the start button, go to the Windows start screen and type "start8" (without quotes) and select it when it appears.)

Select "Unlock Previous Purchase"
http://sd.stardock.com/zubaz/images/start8_unlock1.png
Fill in the appropriate information.

If that fails, select the button below the form labeled "I have a serial number"
http://sd.stardock.com/zubaz/images/Start8_unlock2.png

Select the Register button
http://sd.stardock.com/zubaz/images/Start8_unlock3.png

Select OK
http://sd.stardock.com/zubaz/images/Start8_unlock4.png

Fill out the form and select activate.
http://sd.stardock.com/zubaz/images/Start8_unlock5.png

Two directories will open. One has a file with your activation key and the other is where you will put the license file after it is generated.
http://sd.stardock.com/zubaz/images/Start8_unlock6.png

Use the instructions on http:activate.stardock.com to create the license file.
http://sd.stardock.com/zubaz/images/Start8_unlock7.png
or
http://sd.stardock.com/zubaz/images/Start8_unlock8.png