It has been a couple of years since I encountered a Prolific chipset. However, back when I did see it, their drivers for OS X weren't functional. Some applications were able to communicate, some where. Some hardware worked, some didn't. (I was writing software to communicate with GPS units.)
Even though you were told you bought a Uno, it doesn't mean that's what boot loader was burned on it. This is the problem with the clone makers. Nothing about what they sell is the truth.
The error message you are getting basically says that avrdude (the utility that does the programming) can't communicate with the boot loader. Try using the Duemilanove board type instead of Uno.
Also verify it is actually an ATmega328 and not a ATmega168.