When I encountered this in my development environment it was due to an out of memory condition. Check the php error log (if you have one set up). If not, an easy experiment would be to increase the amount of memory a script is allowed to use. Look for a line in your php.ini file that starts with "memory_limit" and change the value on the right to "32M" without the quotes.
Note that your hosting provider may or may not let you modify this value. I haven't tried it in my production environment yet. Also, if you own the box and php is installed as an Apache module, you may have to issue an "apachectl graceful" as root so that the ini file gets re-read.
Alternately, you can move the file that was installed in includes/extra_configures to includes/modules/YOUR_TEMPLATE so that it only loads the WordPress framework on the page that actually renders WordPress. Make sure the file in includes/modules/YOUR_TEMPLATE is named the same as your main_page variable (e.g. the file should be named wordpress.php if your url is http://yoursite/index.php?main_page=wordpress).
Finally, a quick head's up: I tried that contribution and it didn't seem ready for prime time. It had some issues (you've hit one), and the integration with Zen was pretty much nonexistent. (You'll have two user databases, one for Zen, one for WordPress). I also didn't like the terms of the licensing which stated they wanted a PayPal "donation" for the right to remove their "powered by WoZ" footer. (I'm not sure those terms are permissible under the GPL, so you may be able to disregard it. I elected not to use the contribution so I never did the research.) I've scrapped the contribution and am writing an improved WordPress module from scratch. If you're not willing to do that, then your best bet may be to install the two products separately and modify the WordPress theme to include links to your store. YMMV.
