Are you using a Windows operating system?
If you are, then please download the Photosynth synther from this link:
When it finishes downloading, double click the file to begin the installer and click 'Next' until the installer finishes.
Now you can open the synther from your start menu or by clicking the 'Upload' button on the website.
When the synther opens, sign into it with the email address and password that you signed into the site with.
After signing in, click the 'Start a new synth' button on the synther.
When you see the 'Create synth' window, click the 'Add photos' button to open a file browser, highlight all the photos you want to upload (you can hold down the [Ctrl] key to pick specific photos from a folder or press [Ctrl]+[A] to select all the photos in a folder), and click the 'Open' button.