Some of people have the right kind of keyboard to use console right away, just go to Options->Keyboard/Mouse and Enable developer console.

Others:

Launch L4D2 and go to settings->enable developer console. Go back to Steam interface and right click L4D2 in game library and choose launch properties. Write "-console". Next time console should appear when you start the game.


When you start l4d2 with -console type in : bind "F6" "toggleconsole"
or go to your l4d2 folder (Steam/SteamApps/common/left 4 dead 2) then click left4dead2>cfg and rightclick the config file and run it with Wordpad and search for "bind "´" "toggleconsole" and change it to bind "F6" "toggleconsole" now you can enable it with f6

Thanks to:
JarJar1337