If you are using desktop Internet Explorer on a computer that has the original Photosynth desktop application installed (as you must, if you're uploading photosynths), then you can also check out the version of the Direct3D Photosynth viewer that the site launched with back in 2008.
Find the link to the Direct3D Photosynth viewer in the lower left corner of every photosynth's page. (Naturally, you won't find it on panoramas' pages.)
In the Direct3D Photosynth viewer, you'll notice that the arrows that take you to neighboring photos are situated all around the edge of the viewer. Whenever photos are available, the white arrow outlines will appear and fill in green whenever you hover over them to let you know that you can pan left, right, up, or down... or forward or backward.
For me, though, using the arrows to try to navigate my synths hasn't ever really been all that effective.