Double clicking on the .vpk file will automatically install that addon. If instead of installing, you are prompted to select what program to open the file with select the addoninstaller commonly found here \steam\steamapps\common\left 4 dead 2\bin\addoninstaller
If that does not work or you prefer to do it manually, you can just copy the file directly into the addons directory loacated here \steam\steamapps\common\left 4 dead 2\ left 4 dead 2\addons
Start your game and now that content will be available for you.
You can disable the addons by either deleting them from the folder or going into the extras -> addons menu in game.