I can't find the troll. I know he is supposed to be under a bridge, but I don't know what bride. I've searched everywhere I can think of.

I checked and it says the DLC is installed.

Go to the town's square and read the message board.

Trolls like message boards ;-)

I did, and I got the troll one, but I don't know where he is, and it isn't in my quest log... The contract isn't in my inventory anymore either.

There's a broken stone bridge. There should be a human near it as well. You'll find the troll where you would expect to find trolls that live near a bridge.

Sounds like a weird glitch. Normally, you should be able to double-click on it in the quest log and have a marker lead you to where he is.

I seem to remember it was... up? North...ish. No, that's probably not helping. Sorry.

Ugh, there must be something wrong then, because I have checked there.

Underneath the bridge? Sorry, had to ask.

Head out of town by way of the Merchant area gate, follow the road until you come to the broken bridge. There may be a townsperson there getting his shit kicked in by a Nekker, so save him, and talk to him about the troll (who is just below you, down a couple of ledges and around a corner). This sequence of events may not happen without it in your quest log however, and as for rectifying that problem, I simply have no answers for you.

The troll is pretty hard to distinguish from background environment in evening because of his dark colour. But he is there under the bridge, you need to jump down to the lake.

Yah. I guess there is something wrong with my DLC. Is there a way to uninstall then reinstall the DLC?

Is it installed? Like, it has a green arrow next to it? Again, sorry to have to ask such a question, but I can totally see some people downloading it and thinking that's all they had to do.

Delete "troll.dlc" from My Documents\Witcher 2\Downloads

Did that, my launcher still detects the Troll DLC as being present.

There we go. Deleting it worked. The launcher still showed a check beside it, but I was able to redownload and reinstall. Now the troll is there and the quest works. Thanks guys!