You can change the slideshow order between Photosynth's spatial order and the filename order (which should be the order in which the photos were taken) by using the [Edit Synth] button on your synths when you're signed in. Filename order is only really good for 100% synthy synths, though, otherwise you may wander between different groups of photos, making a complete mess of experiencing as many photos linked to each other as possible. On synths where the author has not enabled 'filename order', but you would like to see what it looks like, you can use the fullstop key [.] to move forward and the comma key [,] to move backwards.
More useful is the ability to change the first photo that is loaded in a synth (also found behind the [Edit Synth] button. This way you can set the first photo to the first image in the largest group for spatial order or the actual first photo that you took when your synth is set to 'filename order'.