when you create a lobby, open the console (usually `) and then use:
mm_dedicated_force_servers ####
Server #1: mm_dedicated_force_servers 217.163.31.65:27015
Server #2: mm_dedicated_force_servers 217.163.30.170:27015
Server #3 (L4D2) : mm_dedicated_force_servers 212.187.208.113:27015
Server #4: mm_dedicated_force_servers 83.142.49.90:28011
Server #5: mm_dedicated_force_servers 83.142.49.90:28010
Press enter then start the game.
The details of the connect commands and some useful admin commands are on:
http://sneakymonkeys.com/admin.txtServer #1 & #2 are public
Server #3 is a public L4d2 server
Servers #4 & #5 are clan servers

p.s. theres another post somewhere about binding these connect commands to a specific key so you don't have to type them in