You're not supposed to play as the zombies. Before you get all technical bitchy, you can by changing your datablock, but being a zombie is not any part of L4B. If they don't move, make sure
1. Zombies are enabled (Zombie minigame menu)
2. You are the Survivor player (Minigame menu)
3. If both 1 and 2 are correct, shoot the zombies. They don't immediately attack.
It's been screwed up since V9. Learn2readthread
It's been screwed up since V10. Learn2readthread before being an starfish.