In order to upload a synth, you must have dragged at least three photos of the same subject from slightly different angles into Photosynth's window and then typed a title into the upper-most left blank in the 'Create a Synth' window.
Once these basic requirements are met, then you should be able to click the [Synth] button.
If you are trying to upload a panorama instead of a photosynth, then use Microsoft ICE to stitch your photos into a panorama and then hand the panorama to Photosynth to upload to your account.
If you use the mobile Photosynth app (which is really actually mobile ICE at this stage) and want to upload your mobile panoramas, you can sign into your Photosynth account inside the app and then select 'Share' to 'Photosynth.net'.
Please ask more questions if I have not answered what you wanted to know.