You probably won't be able to see the synth while in Dreamweaver's editor but save your page after you first embed the synth code and preview it in your browser. It should work right away. Let me know if it doesn't.
Some sites out there (such as Facebook, Live Spaces, and Wordpress) strip out certain html tags, such as the iframe that Photosynth uses. If you're using Dreamweaver, though, I'm assuming that you have your own server space somewhere where you make the rules, not someone else as to what html you can use on your pages. Also, if you run a Wordpress blog on your own server then you could make your own decision there too.