If you happened to read my previous article on playing as infected in Left for Dead single player and tried it out since the Survival Pack came out, you no doubt have cursed my name and damned me to hell. The set of commands in that article are no longer valid.
To get you pouncing in the comfort of your own single player game again, please follow the same steps in the other article, but replace the previous line:
alias "sp_versus" "sv_cheats 1; director_force_versus_start; director_no_human_zombies 0; sb_all_bot_team 1;"
with this one:
alias "sp_versus" "sv_cheats 1; director_force_versus_start; mp_gamemode versus; sb_all_bot_team 1;"