You can only logon to the site as a licensed user. After paying you are given a personal username and password, and you must enter these into the Logon area of the P4 home page in order to gain entry to the site.
It's an easy mistake. The short 'Registration Code' you were told about immediately prior to purchase was simply for use when entering the P4 Registration Page. Use the password you were given when you registered. It appeared in a window when you returned to the P4 site immediately after payment, and it was within the email pro-forma that was presented for you to send to yourself at the same time. Check your email for this.
In this case, please contact us by email, explain the situation, and include your name, the name the purchase was made in (so we can match it to the PayPal account or credit/debit card), and the PayPal 'Receipt ID' for your purchase from the PayPal email. Please ensure you put 'Manual P4 Registration Request' as the Subject of your email. We will then get back to you within a few days after registering you manually to give you your username and password. We'll be as fast as we can, but email is not manned all the time!