Make sure to use the exact information in your registration email. You will have the following info in your registration email:
Email/ID : email@example.com
Serial Code : XXXXX-XXXXX-XXXXX-XXXXX
Enter your Email or ID into box 1 and your serial code into box 2. Don't include the dashes when typing in the serial code. An easy way to get the serial code in is to copy and paste. Go to your email, click the mouse down on the first character in your serial code and drag to the right so that it is highlighted. Then select "Copy" from the "Edit" menu. Run Photos Forever and click "Buy" and then click the mouse in the first edit box on line #2. Right click and select "Paste". (Make sure your Email/ID was entered in line #1) Click "Register" and you're done!
Video: How to Register Mixcraft.