After launching L4D2 it takes a few seconds to the game to load vpk files. While it's doing this you'll see a L4D logo spinning beside the Add-Ons option in the Main menu, wait for the icon to disappear then click the add-ons menu. A list should populate displaying all mods installed.
Also make sure that the .vpk file is placed in "C:SteamSteamAppscommonLeft 4 Dead 2left4dead2addons" and not "C:SteamSteamAppscommonLeft 4 Dead 2left4dead2addonsworkshop".
Of course if eveything above checks out and you're still having a problem please re-download the mod and try it again. Either your download was corrupt or there was some kind of issue with the server when you initially downloaded the file. The Nexus is going through some very big changes right now and there has been a few glitches here and there, especially in newer Nexus pages like here in Left4dead 2 Nexus.
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Also make sure that the .vpk file is placed in "C:SteamSteamAppscommonLeft 4 Dead 2left4dead2addons" and not "C:SteamSteamAppscommonLeft 4 Dead 2left4dead2addonsworkshop".
Of course if eveything above checks out and you're still having a problem please re-download the mod and try it again. Either your download was corrupt or there was some kind of issue with the server when you initially downloaded the file. The Nexus is going through some very big changes right now and there has been a few glitches here and there, especially in newer Nexus pages like here in Left4dead 2 Nexus.