So I downloaded and installed the Troll Trouble DLC, but I can not seem to start the quest, the notice board still says that if you register it is available, any ideas?

I am not sure it is an insta quest. You will have to look for the npc to actually START the quest. Come on, they cant make it THAT easy for ya can they? :P If you found the npc and cant start it, then I am sorry. I have only managed to play for a total of 5 minutes so not been able to see where it starts, or even see where ANYTHING starts for that matter lol

Had the very same problem and got a solution from CD Projekt RED.
Register the game again, using the register.exe in the game folder. Make sure you use CAPITAL letters - lower case does work for the registration, but causes the trouble with the DLC.
If that doesn't work, look here. CD Projekt RED provided another solution including messing around with the registry. (You'll have to scroll down quite a way, keep your eyes open for my Nick (it's Aaden there as well) and you should find it rather quickly)