After entering the product code I get the message: "You are not allowed to install this product". Does this mean that mij version 6.0.5 code cannot be used to install this newer version? Or is there another problem?
After buying PL/SQL Developer you get a license key which will allow you to install all patches/version for the next year. For continued support you'll have to buy a service contract.
You may consider buying a new service contract. Believe me, it is worth it!