The DLC broke the sv_search_key. There's a workaround to connect to the SM servers:
Create a lobby for desired game
DON'T use sv_search_key, instead in the console use:
mm_dedicated_force_servers 217.163.31.65:27015
or
mm_dedicated_force_servers 217.163.30.170:27015
Press 'Start Game' : (This forces the use of one of our 2 servers and is the correct syntax - it does not include quotes as seen on some forums)
However rather than typing this, you can add this as a keybind: click Here and save this file to C:\Program Files\Steam\steamapps\common\left 4 dead\left4dead\cfg (path may be slightly different e.g. Program File x86 etc.,) Each time the game loads it will execute this config and bind a key (in this case F1) to that command. Simply start a game/lobby and just remember to press F1 before you click 'start game'
thx,
MonkeyFiend