I don't believe you can get it to work with Fusion 5. We have talked with Apple about GPT-only Boot Camp installations; I was not aware that they had shipped anything like that yet; the 3TB iMacs shipped with a hybrid MBR/GPT Boot Camp assistant and those are supported in Fusion 5.0.3.
Was this Boot Camp set up by 10.8.4? What is the configuration of your Mac?