The registration code is not needed for a free accounts holder.
When you are prompted to sign into Smilebox, there is a reference to a registration code (as seen below). If you have purchased your subscription online through Smilebox, you can ignore the reference to the registration code, it is not needed. Simply type in your email and password and click "SIGN IN". If you receive any promotion/activation code with specific instruction to enter, then you may have to enter it please.