Hmm, quite bizarre..
I just downloaded the 6.04c German version and compared it to the English version. Apparently Templates/cp_edit_forum.pl does not have the forum_pw code in it, even though the English one does. If you upload cp_edit_forum.pl from the English version temporarily, it should give you access to that field. (The backend code is all there).
CC/Dave/Stefan: why is this missing?