Valerie, I'd love to be able to help, but I don't have any real idea why this is happening to you. I believe that the trial versions are supposed to offer full functionality, but it's possible that CC doesn't want you to be able to build a functioning shop with it. Perhaps someone (Scott? Jay?) can provide a conclusive answer.
Here are a few things I would try, were I sitting at your computer:
Save the shop with a different name and close the program. Restart SCC and open the shop with the new name. Try to export this new shop & see if the .php files are created.
Build a small new shop from scratch, perhaps with just one product -- just enough to try to see if the export will work this time.
Uninstall, clean the registry with Piriform's CCleaner (freeware), then reinstall the trial version and repeat the first two steps.
If all else fails -- see if you can open a help ticket. Explain that you're reluctant to purchase software that you can't seem to make work, but that you really want to resolve this issue so that you can buy it. You have 6 days, and CC support will usually respond very quickly.