Singlocide: Quote" The Witcher 2: Assassins of Kings CD-Key
Using the CD-Key is not required to play, but we recommend you register your copy of The Witcher 2 with it as it grants you access to direct updates, additional language packs, future free DLC and more.
"UNQUOTE
The direct updates are garbage (stick to manual updates—they'll make your life much easier), and language packs/DLC only required registration a looooong time ago. Not anymore.
I actually think the only thing you need to register the game for anymore is to be able to post your arena score on the official Witcher forums. If you choose to submit a score, it shows up on there as an ugly stat thing under your username that shows your rank and makes my eyes bleed.
Anyway, I registered way back on 1.0 so I'm not familiar with how it works now, but do you not have the "register game" option like in the attached picture? Or does that not work for you?