My game came with a piece of paper with a code taped to the box. What´s this for? I´m assuming its for DLC or something, and where would i go about entering the code?

I guess this is for some PreOrder DLC, some retailers did have such special dlcs. For some armor or a extra merchant.

Open the launcher, wait till it connects to the server (little lightening bolt in the upper left hand corner) click downloadable content. put the code in the space provided and click the redeem button.

? It claims my code is invalid?

Restart launcher, code redeem might have gone through server side without client knowing it. That\s what happened with my commando jacket. Patch is also coming soon that should solve DLC issues.

Well, if you can hang in there for a few more hours, the patch "should" be out and will hopefully fix the problem.