I've seen a lot of posts around the net regarding how to get Death Toll & Dead Air Versus maps running. You don't need to download anything, you just need access to a server config. To get those campaigns running in Left 4 Dead, follow these instructions.

Add 2 lines to your server config:

sv_search_key youruniquekey
director_no_human_zombies 0

Reboot your server and you'll be good to go. It will start up with No Mercy VS. Get a bunch of friends together, get into a lobby and type in console (enabled in keyboard/mouse options):

sv_search_key youruniquekey

When you search for a dedicated server, it will choose your server. Once you've joined create the following bind by typing this in console:

bind "F9" "rcon_password yourrconpassword;rcon changelevel l4d_airport01_greenhouse"

bind "F10" "rcon_password yourrconpassword;rcon changelevel l4d_smalltown01_caves"

Now hit F9 or F10 to load those maps and they will load in versus mode.

There are a few bugs associated with doing this that can be slightly painful.

1. Infected climbing paths are sometimes unmarked
2. Sometimes you can go out of the world
3. Players can't join by invite
4. Only one team gets to play the finale

To get players to join, just get them to openserverbrowser in console and search for your server. They should be able to join through the browser though you can sometimes get the server is full message.

That's about it, have fun with the new campaigns.