To mod for L4D2, install the game itself to gain access to files you will be replacing or using and to test mods. You will also need to install Steam because L4D2 requires it.
Click on the "Install Steam" button on the Steam web site About page or on its Home page.
Follow the instructions given.
Open Steam.
Click on LIBRARY menu.
Click on ALL GAMES menu item.
Select Left 4 Dead 2 in the list of games.
Click on the INSTALL button. The INSTALL button is directly under "Left 4 Dead 2" title in the big panel. The figure shows a PLAY button there instead because I have already installed the game.