To get a zombie in your mission open the editor, put a player on the map, than add another unit to the map (name him unitx) and save your mission.
Now "Alt-Tab" out of the game (if you arent running Arma windowed) and open your mission file which can be found in

X:\Documents and Settings\Username\My documents\ArmA\missions

Search the mission file for your placed unit (unitx) and look for the following line :

vehicle="xxxxxxxxx";

Change the xxxxxx into one of the following names :

Civil_Undead_1
Civil_Undead_2
Civil_Undead_3
Civil_Undead_4

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