I highly recommend making your reservation by phone as Ira suggested in his post. The phone number is 011 39 041 520 9070. You do not have to pay until you arrive on the day of the tour. Based on other posts, if you do not prepay on the official web site (www.museiciviciveneziani.it), there is a good chance your reservation did not get recorded.

We reserved by phone for our visit this past May, and everything went smoothly. The operators speak English.