See my post above, but one more post here for tonight and then I'm calling it quits!
After a few more suggestions (thanks Luis) I un-installed Zurek v4, extracted the latest Zwolle v2.5 again and here's what I get for options to install:
Data 000860: Zwolle-St.Michaelis, Schnitger, Demo
OR
Zwolle v.2 demo (not v2.5), and nothing else. It does say in the window to the right that the current version installed is 2.5 and my options are either to re-install or un-install, so obviously I select re-install.
When I attempt to load Zurek v5 I'm still getting the same message at 13% loaded as earlier which is: "Could not open file"....organ installation package 000389 pipe front hw / mixtur 036 C-hbw.
Every file for this set (except for the standard Rotterdam and mini demos) has been downloaded between the last week and last night. So it's one of two things: Either the link to the Zwolle v2.5 download is incorrect and it's not really the v2.5 version, or it looks more likely that I'm missing a rank of pipes from a demo that was updated or added to since I downloaded it.
If someone can tell me which demo organ, package 000389 comes from (I'm guessing it's the Freiberg demo) I can probably solve this mystery in short order.
I've never had any trouble installing an organ before but I guess there's always a first for everything. Keep the ideas coming and thanks for the help!
Marc