If it's advertised as the full version, which it probably is, you have to register with Seriff. This is normal with all their products. If you go to the Help menu there is a registration option. This will give you a phone number to ring. You will be asked for the code that the registration dialog shows you, and will be given another code to enter in the relevant box. You will then have continuous use.
Alternatively you can register at click here Choose Support\Product registration.