[Austin]

Have you tried creating a lobby, selecting the options for the game and then joining your server?

Go to steam / library and right click L4D2 and do a properties
at the bottom under General / LAUNCH OPTIONS
add the force server parm to force the lobby to connect to your server.
Set the IP and port to match your server.

for example
+mm_dedicated_force_servers 10.10.10.10:27015
Then run L4D2, create a lobby with the type of game you want and then go.

NOTE:
The lobby will NOT connect to the server and it will not give you any error message
if anyone is already on the server

You have to kick everone off before connecting with the lobby.