Keys are provided by your retailer (eg Steam, Amazon, GamersGate), so please contact them if you can't find forum registration keys. Paradox do not directly distribute CD (forum registration) Keys, they are always the responsibility of the vendor.
Games bought from our Webshop come in the form of Steam Activation keys which cannot be used for forum registration; once activated on Steam the CD Key link there is where you get your forum registration key.
2016 UPDATE: we are moving away from the key system for games on Steam. The CD Key link may well be empty now. Instead link your Paradox account to your Steam account here
then go here
and hit the Sync Owned Items button. All Paradox games you have on Steam will be registered automatically. Non-Steam games will still use the Registration Key system.