Hi there, John,
::1) What operating system are you using?
::2) Where are you seeing the "... Photosynth doesn't work on your operating system yet." message? I am assuming it is here on the Photosynth.net website.
If you are running the Windows Developer Preview ("Windows 8"), then:
::A) Here is the link to download the Photosynth app for Windows: http://photosynth.net/installer/photosynthinstall.exe
After you save it on your computer, open the installer and click 'Next' until it finishes.
After it finishes installing, you can open the Photosynth app from your Windows Start Menu/Start Screen. You can also create a desktop icon for the app or launch it (after it is installed) from the 'Upload' button here on the website.
When the Photosynth app opens, and you sign into your account, you can drag multiple photos from Windows Explorer, directly into the Photosynth app. Give your photosynth a title (upper left corner of 'Create a Synth' window) + 'Synth'.