The guide for Witcher 2 which comes with the premium/collector's edition says you can get the troll contract off the notice-board, but I never got any.
Do I have to read the notice to get the quest? (And what should I do if I've sold off those notices?)
CD Projekt says they will address the Troll DLC problem in the next patch specifically here.
- Fixed the free Troll Trouble DLC download. It is now included n the patch so there's no need to download and install it separately
They claim the patch could be available as early as the evening of the 25th (which is like right now).
If you're playing the Steam version, I image you'll have to wait until the patch is rolled into an auto update.
Keep in mind that if you advance beyond chapter 1, the troll quest won't be available on your most current file (at least I can't see how it could be).
The Troll quest is initiated by taking the notice from the board
You don't need to keep the notice or read it, taking it from the notice board is enough to get the quest started.
The Troll quest is free downloadable content (DLC), it does not come directly with the game but you have to register the game and download it separately. If the notice wasn't there, probably something went wrong during download or installation of the DLC (see the guide from CD Project on activation and DLC). One thing I found counter-intuitive is that after downloading the DLC you have to go back to the DLC dialog and click on install, it doesn't install automatically after downloading.
