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. Look for the entry labeled Left 4 Dead 2 – Add-on Support click and install it. Just go into steam and open up the Tools tab.
PLAYERS: If you are not looking to design but play custom campaigns all you need to do is install the add-on support package.
