Installing add-ons

For default L4D2 installs:
Unzip download to your desktop

Open my computer and navigate to
c:program files (x86)/steam/steamapps/common/left 4 dead 2/left4dead2/addons

Then cut downloaded and unzipped .vpk file and paste it into the addons folder.

Now you are good to go.

To manage your add-ons ingame:
At main menu click extras and then add-ons
To turn on the add-on make sure it has a check mark next to it.
To turn off the add-on simply remove the check mark.