This only works in single player, and is of course an incomplete feature. There are a few posts going around, but people seem confused.
There are scraps of Versus mode inside the demo. If this post is somehow against the rules, delete it at your leisure!
In options, you need to enable the developer console command.
Start a singleplayer game.
Open your console with the ~ key.
Type "sv_cheats 1"
Type "ent_fire !self setteam 3" to join the Infected team
You get to choose where to spawn as a baddie, but the AI controlled survivors will not leave their starting room and the game will not start on its own.
Type "director_force_versus_start 1" in your console to begin the round.
The survivors will start hacking their way through the apartment building and you finally get to try your zombie character!
When you die (and you probably will), you will not respawn. You must use these commands in the console.
z_spawn tank
z_spawn boomer
z_spawn smoker
z_spawn hunter
These will spawn you as that infected at the spot you are currently "flying" as a spectator.
I was iffy about my pre-order to be honest, but now that I've gotten 3 of my friends to try the zombies, they're all buying the game today!
Have fun