just unzip the file and run the installer.
When complete it will be under "left 4 dead 2 Add-On Installer" in the start menu
when it starts it will ask you to set your install folder.
The auto detect folder should work. if it doesnt you need to click browse.
The Folder you need to find is /steamapps/common/left 4 dead 2
hope fully it should then work and open the main screen.
Using the L4D2AOI
Any addons already installed will be under the "Installed" tab. If any of them are highlighted pink, it means that the addon isn't currently managed by my program.
You can right-click installed addons to view their properties.
if you click on the "Availible" tab it should list the addons you can install.
You can multi-slect Availible addons to install more than one at a time.
if you right click an Availible addon and select install it will ask you to confirm the install, hit ok.
you might get a UAC warning about running AOI.exe, you need to press ok for the install to happen.
The AOI.exe is the program that actually installs the addon (on systems > windows xp, steam by default is installed into a folder that requires admin access).
It Should then download and install the addon for you.
The program should download and install the addon for you. At the moment you have to open and reclose the program for the "Installed" tab to register that you have installed a new addon, but the Addon should actually be installed and usable.
Most of the addons require that you also install "L4D1 Extra Assests" & "New Infected For L4D Maps"