Since some of you haven't really touched Left 4 Dead 2 often, or custom content rather, you need to make sure you have Left 4 Dead 2 Addon Support installed. You can find this under the tools tab in Steam. After that, any custom campaigns/maps you download will 99% of the time be in a VPK format. These VPK files go into your addons folder in your L4D2 folder. After you extract them there, make sure to double click them. If things have gone correctly, it should say your file installed successfully. That should be it. If there's any problems, make sure you don't have any previous versions of that campaign/map installed. Just delete the VPK file out of the folder and it should be uninstalled. PS: You don't need to install any addon support things for L4D1. Just put the VPK in the same spot in your L4D1 folder.