There's a Downloads link at the top of the forums. You'll find the campaign and map download links there. To install the campaigns unzip the "vpk" file to you pc, then double-click the file. This should cause steam to install it for you.
To manually install, extract or copy the vpk file to "<your steam install path>\steam\steamapps\common\Left For Dead 2\leftfordead2\addons" you might have to create the "addons" folder if this is your first addon.
For example my files are located at: C:\Program Files (x86)\Steam\steamapps\common\left 4 dead 2\left4dead2\addons
If you have trouble let me know and I'll see if I can help.
