If you're using All-inclusive or Premier versions of the software, you will need to contact the developers in order to receive a serial key. If you're using the free version, firstly, you will need to make sure that you're using the updated program. If the antivirus is up to date, launch it and click the Registration button from the Settings menu:
After that, press Register now and follow the on-screen instructions. For further information, I recommend you check the official Support page.