taken from LDn's gaming communityTo join the LDN server, start the game. Click on options, then click on keyboard/mouse and toggle the Allow developer console to "Enabled", click on done. Once your back at the home screen press the`Key(AKA the key to the left of the number 1 key) that should open up the console. Once the console is open type this to connect to the server:
server ip :
For more information about getting l4d2 up and running see their thread , since warez arnt allowed i cant post it here ..
so check the source. http://www.pakgl.com/forum/thread619.html