Addons & common problems with adding them can be found here. http://www.nofearcomputers.com/downloads/games/l4d2/l4d2downloads.htm

Steam cloud in left 4 dead 2 menu bug, Cant click anything
When you start Left 4 Dead 2 ...Steam cloud in left 4 dead 2 menu bug, Cant click anything...Left 4 Dead start screen is locked can't click anything.
This is a first use problem / bug, still around :(

The FIX, In Program Files\Steam\steamapps\common\left 4 dead 2\left 4 dead 2\ REMOVE but save your addons and cfg folder.
If you have anything added to your "Set Launch Options" remove that also (but save it) Shut down & restart steam, start Left 4 Dead 2.
Then shut Left 4 Dead down, close steam complete exit, put your addons and cfg folders back, Restart Steam, Right click Left 4 Dead 2,
select properties, add your "Set Launch Options" back, restart Left 4 Dead all should be good.

Running source - meta mods add -insecure sv_consistency 0 to your "Set Launch Options" You won't be allowed to join regular VAC steam servers.

Turn off the start videos add -novid  to your "Set Launch Options"