PLAYERS: If you are not looking to design but play custom campaigns all you need to do is install the add-on support package. Just go into steam and open up the Tools tab. Look for the entry labeled Left 4 Dead 2 - Add-on Support click and install it. This download is sizable, but will update your L4D2 installation to allow running ANY map, including custom maps made for the original Left 4 Dead. Once installed, you do not need to run this application, it will always be active.
We will be updating the Left 4 Dead 2 game UI to include links to this package when applicable but for now please just download it directly. If you see any missing textures or models from your favorite map let us know in the forums.
DESIGNERS: Besides also downloading the above file you will need to download the Left 4 Dead 2 - Authoring Tools. Downloading it will add applications, utilities, and example files that are useful for building L4D2 game add-ons. These are similar to the tools released for Left 4 Dead, but include many updates and new features.
You can read more about how to get the SDK up and running at the official L4D2 Blog.