fdident versions 3.0f and earlier need an additional new file to be able to run properly under Matlab 6.x.
Please go to directory <matlabroot>\toolbox\fdident\fdident\@iddat,
and create a file numel.m there with the following contents:
function ne2 = numel(dat,varargin)
ne2=1;
The version on the WEB server already contains this file,
so an alternative is to upgrade fdident from the WEB.