Howdy, 'tain't likely,
All you need is to be signed into your account (as you were to post this question). The comment box is below the photosynth or panorama on the right side of the page.
The one exception that I can think of is that on iOS devices you don't, currently, see the normal 'view' page, but rather are intercepted and sent to an HTML panorama viewer. Because of this current design, it is not possible to comment on photosynths or panoramas from Mobile Safari, for the time being.
If you use any other mobile device, however, (Windows Phone, Android, Blackberry, etc.) or a desktop browser, such as Internet Explorer, Opera, Safari, Chrome, or Firefox on Windows, MacOS, or Linux you should see the comment box straight away.