Norton Anti-Virus for Mac - two trojans detected

I have a mac and following the recent news reports regarding the infection of macs with viruses hidden in Flash Updates, I carried out a scan of my mac using my Norton Anti-Virus Software.

 

The scan detected two Trojans, I clicked repair, but nothing seemed to happen and a later scan found the viruses still on my mac.

 

What can I do now, how badly is my mac infected and why didn't the Norton Anti Virus Software prevent the infection?