Try to be more specific. What do you mean by saving the file?
Your synth is saved by photosynth until you delete it on your page. If you want to change certain details of your synth and save those changes, you must click the " edit " button when you view you synth. Then you can save any changes you make when you edit it. As far as saving your synth onto your harddrive to have as a file seperate from photosynth, I am not sure you can do this.
There might be a way, but I am not qualified to say if there is or not.
I think it would help if you asked exactly how you want to save the file and I am sure that someone will be able to answer your question.