Make sure to use the exact information in your registration email. You will have the following info in your registration email:
Registration ID : email@example.com
Registration Code : XXXXX-XXXXX-XXXXX-XXXXX
Enter your Registration ID and your Registration code in the same named boxes. Don't include the dashes when typing in the registration code.
An easy way to get the registration code entered is to use "copy and paste".
Here is how to use copy and paste: Select the code. To do this, place the cursor just before the code. Left click the mouse. While holding the left mouse button down, drag the mouse over the code (it is highlighted). Release the left click/mouse button (the code is still highlighted).
Next, right click the mouse, while the code is highlighted. From the right click menu (right click the mouse), select "copy".
Open the software. Click the "enter code" button when the software first opens or find it from the drop down menu under Help. Skip buying the software (since you have already purchased). Place your cursor in the code entry spot of the software (the cursor is blinking). Right click the mouse and select "paste" from the right click menu. Your code is "pasted" into the code entry box. Make sure there is not a space before the code.
Click "Register" and you're done!
Video: How to Register Mixcraft.