Hi,
I don't know if you still searching the answer for your problem but you need to add com.apple.Boot.plist file in Extra folder. I'm assuming you're using Boot-132 USB, right? Make sure you specify "Default Partition" in com.apple.Boot.plist file. "Default Partition" is referring to your Mac OS X partition, for example:-
<key>Default Partition</key>
<string>hd(0,2)</string>
Base on the above example, the Mac OS X was installed at the second partition. Change it according to your setup.
kizwan