Zombies / robots (in Splat Pack) are still in game. You just need to open .conf file in notepad and add -robots at the end.
Example:

(dosboxCARMA.conf)
voodo2c.exe -novoodoo -robots

(dosboxCARMA3dfx.conf - for hardware version)
voodo2c.exe -vrush -robots

If you installed Carmageddon to protected location (like Program Files), remember to edit files with administrator privileges, or else the game may not read changes properly.