A couple of things. First, I purchased a train ticket on Trenitalia a few days ago. Everything worked fine with my American credit card.
If you can log on and have the system recognize your ID and password, you're half way there. Also, you might encounter difficulties if you try for the discounted fares.
Are you trying to use the home delivery option? It's available on some runs. This one is tough. I haven't broken the code on this one because the fields are in Italian which I can't translate. If the journey is entirely in Italy, you should be presented with a self-service option and two ticketless options. If neither your origination point for destination is equipped with a self-sevice machine, you won't see that option.
Don't be afraid of these options. I selected the self-service option. My receipt contains a reservation code that I can punch into any self-service machine in Italy to get my ticket. On the ticketless, the conductors carry PDA devices. They can enter your reservation code and validate everything by wireless.
Last resort. Take a look at www.italiarail.com. They are the official site in North America for rail travel in Italy. They are a broker.
Finally, most North Americans are so insistent about having their tickets in hand before they travel to Italy. They are so fearful that there won't be a train when and where they want to go and if there is, it will be full. Nothing can be farter from reaility. Trenitalia has 8,000 train runs a day in Italy. Except for major holidays, you will always find a train and a seat. Exceptions can be found in the Cinque Terre during high seat. The trains are like Disneyland. But, you just get on and stand. It's only a few minutes the next stations.