I went to the website http://www.museiciviciveneziani.it/frame.asp?sezione=prenotazioni and called the number (call center), hoping that they would make reservations for Doge's palace and I could purchase the pass later. But they said I'll need to purchase the card and no need to make reservations then.
Everywhere else, like Accademia, Villa Borghese, I've been able to call and make reservations.
Can some one give me the site of Doge's palace (yes, I could not find it! Its disturbing.) or the phone number to make the call? Or, do I really need reservations for Doge's palace? What if I just buy the San Marco Museum Plus Pass and go there?