OK, I've solved my problem...
The missing 32 bit module (libgthread-2.0.so.0) is in Debian package ia32-libs-gtk. Installation of this package solved the problem and help now works.
In searching forum threads, looking for a solution, I found several where the author assumed that one could invoke the search function of Synaptic (using the module name) and find the package which includes the module. That does not work. Synaptic will show you module names only for installed packages -- not much of a help since the problem is that the package is not installed. Further, even if the package is installed, there is no "search" other than eyeballing the (very long) list or copy/patching it into a text editor where you have a search capability.
You need to go to http://debian.org/distrib/packages
and use their "search the contents of packages".
To complete the "installation", I've also gone into the KDE control center, KDE Components, File Associations, and added the association between *.dxf files and qcad.
I don't know -- it seems to me that a commercial product shouldn't require this sort of hacking.