Another person and I tried to do the zombie fest #4 ( be the zombie mode ) but we couldn't do it. Is there a particular way of doing it? If anybody has done it on gog please let me know.

I'm playing with "Invasion" on, I thought it would choose a Zombie at random. I could be wrong. I think it's also based on the frequency of the Invasion, maybe you should try setting it to every night, instead of every three nights.

(i know its ultra necromancy but...)
if you want to play "be the zombie" you first need atleast one other player AND
IF you want to be THAT zombie you need to connect to sesion with enabled invasion what is both ingame night AND that night is open for invasion (depends on invasion friequency set by sesion host)
OR
IF you want to be on survivor side (as usual) you need either to host a game sesion with invasions enabled and set to your prefeared prequency of invasions OR stay in/join a sesion in whitch host has enabled invasions AND wait for "zombie" player to connect (and he will try to see whats inside of you and other survivors (LITERALLY) )