Once the .VPK file is downloaded, simply double click it to begin automatically installing the files. It will copy itself to the appropriate Steam folder.
If there is any issue with this process, you can manually install the add-on by moving or copying the .VPK to the Left 4 Dead Add-ons folder.
This folder is typically located here:
C:Program FilesSteamsteamappscommonleft 4 deadleft4deadaddons