It is not necessary to enable Trojan Horse Protection in VirusBarrier X6 if you only run Mac OS X on your computer. The VirusBarrier Threat Filters and Real-Time scanner protect your computer from Trojan Horses. This feature does recognize the actions of Windows Trojan horses, so if you are running Windows in virtualization - with a program such as VMware Fusion or Parallels Desktop - and sharing your Mac's Internet connection in NAT mode, you'll be protected.
To Enable Trojan Protection, click the Firewall button at the top of the VirusBarrier X6 window, click the Trojan tab, slide Trojan Horse Protection to On, and click Enable All in the lower left corner of the window. All of the Trojans in the list should be checked.