I use prepaid credit cards on PSN all the time. Did you make sure you registered it online? PSN, and most other purchases online will require you to register the card (which the information for that would be with the little instruction thing that comes with the card). You gotta go to the card's website and put in your name, an address and a phone number, then you will be able to do online purchases, such as PSN when you use that same information (it doesnt even have to be right, it just has to match with what you put in when you register the card).
You can also use the online site to look at your active balance on the prepaid card.
And i have used both prepaid visas and mastercards on PSN and other online purchases.